diff --git a/lectures/Day_4.ipynb b/lectures/Day_4.ipynb
index eab1520..9022de9 100644
--- a/lectures/Day_4.ipynb
+++ b/lectures/Day_4.ipynb
@@ -91,7 +91,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "print(sorted(['2000', '30', '100']))"
+    "print(sorted(['2000', '30', '100', '5', '301']))"
    ]
   },
   {
@@ -104,7 +104,7 @@
    },
    "outputs": [],
    "source": [
-    "ord('2')"
+    "ord('3')"
    ]
   },
   {
@@ -476,7 +476,7 @@
    },
    "outputs": [],
    "source": [
-    "bool('')"
+    "bool([])"
    ]
   },
   {
@@ -796,7 +796,7 @@
     "HOST = 'global'\n",
     "\n",
     "def show_host():\n",
-    "    print(f'HOST inside the function = {HOST}')\n",
+    "    print('HOST inside the function = {HOST}')\n",
     "\n",
     "show_host()\n",
     "print(f'HOST outside the function = {HOST}')\n"
@@ -824,10 +824,12 @@
     "def change_host():\n",
     "    HOST = 'local'\n",
     "    print(f'HOST inside the function = {HOST}')\n",
-    "\n",
+    "def app2():\n",
+    "    print(HOST)\n",
     "print(f'HOST outside the function before change = {HOST}')\n",
     "change_host()\n",
-    "print(f'HOST outside the function after change  = {HOST}')\n"
+    "print(f'HOST outside the function after change  = {HOST}')\n",
+    "app2()"
    ]
   },
   {
@@ -895,7 +897,7 @@
     }
    },
    "source": [
-    "Will the global variable never to changed by function?"
+    "### Will the global variable never to changed by function?"
    ]
   },
   {
@@ -957,7 +959,7 @@
     "\n",
     "count1 = cytosine_count('CATATTAC')\n",
     "count2 = cytosine_count('tagtag')\n",
-    "print(count1, count2)"
+    "print(count1, \"\\n\", count2)"
    ]
   },
   {
@@ -1033,8 +1035,8 @@
     "def foo():\n",
     "    do_nothing = 1\n",
     "\n",
-    "r = foo()\n",
-    "print(f'Return value of foo() = {r}')"
+    "result = foo()\n",
+    "print(f'Return value of foo() = {result}')"
    ]
   },
   {
@@ -1048,6 +1050,67 @@
     "- Use `return` for all values that you might want to use later in your program"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {
+    "cell_marker": "'''",
+    "slideshow": {
+     "slide_type": "slide"
+    }
+   },
+   "source": [
+    "### Small detour: Python's value for missing values: `None`\n",
+    "\n",
+    "- Default value for optional arguments\n",
+    "- Implicit return value of functions without a `return` statement\n",
+    "- `None` is `None`, not anything else"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "None == 0"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "None == False"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "None == ''"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "bool(None)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "type(None)"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {
@@ -1287,12 +1350,12 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "def format_sentence(subject, value, end):\n",
+    "def format_sentence(subject, value = 13, end = \"....\"):\n",
     "    return 'The ' + subject + ' is ' + value + end\n",
     "\n",
     "print(format_sentence('lecture', 'ongoing', '.'))\n",
     "\n",
-    "print(format_sentence('lecture', 'ongoing', end='!'))\n",
+    "print(format_sentence('lecture', '!',  value='ongoing'))\n",
     "\n",
     "print(format_sentence(subject='lecture', value='ongoing', end='...'))"
    ]
@@ -1345,7 +1408,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "def format_sentence(subject, value, end='.', second_value=None):\n",
+    "def format_sentence(subject, value, end='.', second_value=0):\n",
     "    if second_value is None:\n",
     "        return 'The ' + subject + ' is ' + value + end\n",
     "    else:\n",
@@ -1357,67 +1420,6 @@
     "                      second_value='self-referential', end='!'))"
    ]
   },
-  {
-   "cell_type": "markdown",
-   "metadata": {
-    "cell_marker": "'''",
-    "slideshow": {
-     "slide_type": "slide"
-    }
-   },
-   "source": [
-    "### Small detour: Python's value for missing values: `None`\n",
-    "\n",
-    "- Default value for optional arguments\n",
-    "- Implicit return value of functions without a `return` statement\n",
-    "- `None` is `None`, not anything else"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "None == 0"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "None == False"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "None == ''"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "bool(None)"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "type(None)"
-   ]
-  },
   {
    "cell_type": "markdown",
    "metadata": {
@@ -1520,9 +1522,7 @@
     "\n",
     "- Notebook Day_4_Exercise_1  (~30 minutes)\n",
     "- Go to Canvas, `Modules -> Day 4 -> Exercise 1 - day 4`\n",
-    "- Quiz. Go to Canvas, `Modules -> Day 4 -> PyQuiz 4.1`\n",
     "\n",
-    "- Lunch break\n",
     "\n",
     "- Extra reading:\n",
     "    - https://realpython.com/python-kwargs-and-args/\n",
@@ -1636,7 +1636,7 @@
    "source": [
     "import os\n",
     "\n",
-    "os.system(\"date\")"
+    "os.system(\"ls\")"
    ]
   },
   {
@@ -1648,7 +1648,7 @@
     }
    },
    "source": [
-    "How to find the right module and instructions?\n",
+    "### How to find the right module and instructions?\n",
     "\n",
     "- Look at the [module index](https://docs.python.org/3/py-modindex.html) for Python standard modules\n",
     "- Search [PyPI](http://pypi.org)\n",
@@ -1689,7 +1689,8 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "text = 'Programming,is,cool'"
+    "text = 'Programming,is,cool'\n",
+    "text.split(sep=',')"
    ]
   },
   {
@@ -1937,7 +1938,7 @@
    "source": [
     "def process_file(filename, chrom, pos):\n",
     "    \"\"\"\n",
-    "    Read a vcf file, search for lines matching\n",
+    "    Read a very large vcf file, search for lines matching\n",
     "    chromosome chrom and position pos.\n",
     "\n",
     "    Print the genotypes of the matching lines.\n",
@@ -2116,7 +2117,10 @@
     }
    },
    "outputs": [],
-   "source": []
+   "source": [
+    "from files import mywork\n",
+    "mywork.pipeline([\"accctt\", \"gaccct\"])"
+   ]
   },
   {
    "cell_type": "markdown",
@@ -2134,6 +2138,13 @@
     "https://www.python.org/dev/peps/pep-0008/?#comments"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Lunch"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {
@@ -2142,13 +2153,11 @@
     }
    },
    "source": [
-    "### Quiz time!\n",
+    "### Quiz time\n",
     "\n",
     "Go to Canvas, `Modules -> Day 4 -> PyQuiz 4.1`\n",
     "\n",
-    "~10 min\n",
-    "\n",
-    "Lunch after quiz"
+    "~10 min"
    ]
   },
   {
@@ -2404,15 +2413,6 @@
     "#### Orange tree data"
    ]
   },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "!cat ../downloads/Orange_1.tsv"
-   ]
-  },
   {
    "cell_type": "code",
    "execution_count": null,
@@ -2519,7 +2519,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "df.std()"
+    "df.max()"
    ]
   },
   {
@@ -2578,7 +2578,8 @@
    },
    "outputs": [],
    "source": [
-    "df.age"
+    "df_new = df.age\n",
+    "df_new "
    ]
   },
   {
@@ -2680,7 +2681,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "df.loc[[1, 3]] # select row 2 and 4"
+    "df.loc[[1, 3, 0]] # select row 2 and 4"
    ]
   },
   {
@@ -2709,7 +2710,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "df.loc[[1, 3], ['age', 'height']]"
+    "df.loc[[0], ['age']]"
    ]
   },
   {
@@ -2755,7 +2756,7 @@
    },
    "outputs": [],
    "source": [
-    "df.loc[1:3, ['age']].mean()"
+    "df.loc[1:10]"
    ]
   },
   {
@@ -2801,6 +2802,7 @@
    "source": [
     "import math\n",
     "df['radius'] = df['circumference'] / 2.0 / math.pi\n",
+    "\n",
     "df"
    ]
   },
@@ -2836,7 +2838,7 @@
     "    'age': [1,2,3,4],\n",
     "    'circumference': [2,3,5,10],\n",
     "    'height': [30, 35, 40, 50]\n",
-    "})"
+    "})\n"
    ]
   },
   {
@@ -2885,7 +2887,7 @@
    },
    "outputs": [],
    "source": [
-    "pd.concat([df1, df2], axis=1)"
+    "pd.concat([df2, df1], axis=0)"
    ]
   },
   {
@@ -2914,17 +2916,6 @@
     "### Slightly bigger data frame of orange trees "
    ]
   },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {
-    "scrolled": true
-   },
-   "outputs": [],
-   "source": [
-    "!head -n 10 ../downloads/Orange.tsv"
-   ]
-  },
   {
    "cell_type": "code",
    "execution_count": null,
@@ -2936,7 +2927,7 @@
    "outputs": [],
    "source": [
     "df = pd.read_table('../downloads/Orange.tsv')\n",
-    "df.iloc[0:5]  # can also use .head()"
+    "df.head(3)  # can also use .head()"
    ]
   },
   {
@@ -2971,9 +2962,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "metadata": {
-    "scrolled": true
-   },
+   "metadata": {},
    "outputs": [],
    "source": [
     "df[df.age > 500]"
@@ -2985,7 +2974,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "df[(df.age > 500) & (df.circumference < 100)]"
+    "df[(df.age > 500) & (df.circumference < 100) ]"
    ]
   },
   {
@@ -3026,7 +3015,7 @@
    },
    "source": [
     "### Small exercise 1\n",
-    "* find the maximal circumference and then filter the data frame by it"
+    "* Find the maximal circumference and then filter the data frame by it"
    ]
   },
   {
@@ -3035,7 +3024,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "df[df.circumference == df.circumference.max() ]"
+    "df"
    ]
   },
   {
@@ -3173,7 +3162,7 @@
    },
    "outputs": [],
    "source": [
-    "small_df.plot(x='age', y='height', kind='line') # plot the relationship of age and height\n",
+    "small_df.plot(x='age', y='circumference', kind='line') # plot the relationship of age and height\n",
     "# try with other types of plots, e.g. scatter"
    ]
   },
@@ -3186,7 +3175,7 @@
     }
    },
    "source": [
-    "### What if no plots shows up?"
+    "### Tips: what if no plots shows up?"
    ]
   },
   {
@@ -3296,7 +3285,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "small_df.plot(kind='hist', y = 'age')"
+    "small_df.plot(kind='hist', y = 'age', fontsize=18)"
    ]
   },
   {