Add session4 materials and align session2 solutions structure

Author: steliosot

README.md

@@ -21,9 +21,14 @@ You can also pull updates from the original repository (upstream) when new mater
 
 ## Sessions
 
-- [Session 1 README](./session1/README.md)
-- [Session 2 README](./session2/README.md)
-- [Session 3 README](./session3/README.md)
+- [Session 1 README](./session1/README.md)  
+  Setup, Python fundamentals, loops/indexing, and CSV basics.
+- [Session 2 README](./session2/README.md)  
+  `csv.DictReader`, key-based data access, and practical data cleaning flow.
+- [Session 3 README](./session3/README.md)  
+  Iterators/generators, streaming vs loading, complexity, and intro RAG concepts.
+- [Session 4 README](./session4/README.md)  
+  Serial vs multiprocessing basics, process management, and `Pool`-based parallel image processing.
 
 ## Notes
 

session2/README.md

@@ -15,12 +15,14 @@ By the end of Session 2, you should be able to:
 2. [Part 2](./session-02-part-02.md)
 3. [Part 3](./session-02-part-03.md)
 4. [Homework](./session-02-homework.md)
-5. Review your [solutions](./solutions/) after attempting tasks yourself.
-6. Practice with [quizzes](./quizzes/) when ready.
+5. Write your own work in [solutions](./solutions/).
+6. Review [reference solutions](./session_solutions/) only after attempting tasks yourself.
+7. Practice with [quizzes](./quizzes/) when ready.
 
 ### Notes
 
 - Keep your own solutions in separate files inside `solutions/`.
 - Use exercise-style names in `solutions/` (for example `exercise-02-01.py`, `exercise-02-02.py`).
+- Reference answers are in `session_solutions/`.
 - Update this `README.md` with progress and notes.
 - Do not commit `.venv/` to GitHub.

session2/session-02-part-02.md

@@ -217,7 +217,7 @@ with open("movies.csv", "r") as file:
 
 > [!TIP]
 >
-> What are the time and space complexities of this script? See solutions also
+> What are the time and space complexities of this script?
 >
 > <details>
 > <summary>Show answer</summary>

session2/session_solutions/README.md

@@ -0,0 +1,18 @@
+# Session 2 Reference Solutions
+
+This folder contains reference solution code provided by Stelios.
+
+Write your own code first in `session2/solutions/`.
+Use these files only to check your approach after you complete each task.
+
+## Files
+
+- `exercise-02-01.py`: Part 2 solution script (DictReader practice + incomplete CSV handling)
+- `exercise-02-02.py`: Part 3 solution script (mini dictionary clean + CSV cleaning flow)
+- `exercise-02-homework.py`: Homework solution script (Gemini helper + AI-assisted dataset cleaning)
+- `movies_clean.json`: output generated by the mini dictionary example in `exercise-02-02.py`
+
+## Notes
+
+- Quiz content is intentionally excluded.
+- Run scripts from the `session2` folder so dataset-relative paths work.

session2/solutions/exercise-02-01.py …ion2/session_solutions/exercise-02-01.pysession2/solutions/exercise-02-01.py renamed to session2/session_solutions/exercise-02-01.py session2/solutions/exercise-02-01.py renamed to session2/session_solutions/exercise-02-01.py

@@ -1,25 +1,11 @@
 # Solutions
 
-Use this folder to save your solutions when working locally.
+Use this folder to save your own Session 2 solutions when working locally.
 
-## Files
+Suggested files:
 
-- `exercise-02-01.py`: Part 2 solution script (DictReader practice + incomplete CSV handling)
-- `exercise-02-02.py`: Part 3 solution script (mini dictionary clean + CSV cleaning flow)
-- `exercise-02-homework.py`: Homework solution script (Gemini black-box helper + AI-assisted dataset cleaning)
-- `movies_clean.json`: output generated by the mini dictionary example in `exercise-02-02.py`
+- `exercise-02-01.py`
+- `exercise-02-02.py`
+- `exercise-02-homework.py`
 
-## Homework solution
-
-- File: `exercise-02-homework.py`
-- Run from `session2/`:
-
-```bash
-python3 solutions/exercise-02-homework.py
-```
-
-- Output file: `studio_ghibli_movies_ai_clean.csv`
-
-## Notes
-
-- Run scripts from the `session2` folder so dataset-relative paths work.
+This folder is intentionally empty except for this README.

session2/solutions/exercise-02-02.py …ion2/session_solutions/exercise-02-02.pysession2/solutions/exercise-02-02.py renamed to session2/session_solutions/exercise-02-02.py session2/solutions/exercise-02-02.py renamed to session2/session_solutions/exercise-02-02.py

@@ -0,0 +1,32 @@
+## Welcome to Session 4
+
+### Learning goals
+
+By the end of Session 4, you should be able to:
+
+- explain serial execution vs process-based parallel execution
+- use `multiprocessing.Process` with `start()` and `join()`
+- use `multiprocessing.Pool` for cleaner parallel workflows
+- compare serial vs parallel performance on practical tasks
+
+### Recommended order
+
+1. [Part 1](./session-04-part-01.md)
+2. [Part 2](./session-04-part-02.md)
+3. Write your own work in [solutions](./solutions/).
+4. Review [reference solutions](./session_solutions/) only after attempting tasks yourself.
+5. Practice with [quizzes](./quizzes/) when ready.
+
+### Notes
+
+- Tutorial and warm-up material is included directly inside each part markdown file.
+- Keep your own solutions in separate files inside `solutions/`.
+- Use exercise-style names in `solutions/` (for example `exercise-04-01.py`, `exercise-04-02.py`).
+- Reference answers are in `session_solutions/`.
+- Use `if __name__ == "__main__":` in multiprocessing scripts.
+- If `requirements.txt` is missing, create it and add:
+  - `requests==2.32.3`
+  - `Pillow`
+  - `quizmd`
+- Keep all dependencies in `requirements.txt`, then install with:
+  - `pip install -r requirements.txt`