diff --git a/Semaine_04/Bibliotheque/functions.py b/Semaine_04/Bibliotheque/functions.py
index 56c6a6a..c79e3ef 100644
--- a/Semaine_04/Bibliotheque/functions.py
+++ b/Semaine_04/Bibliotheque/functions.py
@@ -5,6 +5,7 @@ def add_new_book(book_list: list[dict[str, str]], book: dict[str, str]): # On pr
     if book in book_list:
         return f"{book["Title"].upper()} y est déjà."
     book_list.append(book)
+    save(book_list)
     return f"{book["Title"].upper()} a été ajouté."
 
 def take_out_book(book_list: list[dict[str, str]], title: str):
@@ -12,6 +13,7 @@ def take_out_book(book_list: list[dict[str, str]], title: str):
     for book in book_list:
         if book["Title"].lower() == title.lower():
             book_list.remove(book)
+            save(book_list)
             return f"{title.upper()} a été retiré."
     return f"{title.upper()} n'y est pas."
 
@@ -39,7 +41,7 @@ def search_book_by_letter(book_list: list[dict[str, str]], letter: str):
     """Cherche un livre par lettre de départ"""
     results = []
     for book in book_list:
-        if book["Title"].startswith(letter):
+        if book["Title"].lower().startswith(letter):
             results.append(book)
     return results if len(results) > 0 else "Pas de résultat."
 
@@ -53,4 +55,4 @@ def save(book_list):
 def load():
     """Récupère le contenu d'un fichier JSON"""
     with open("books.json") as json_file:
-        return json.load(json_file)
\ No newline at end of file
+        return json.load(json_file)
diff --git a/Semaine_04/Jour_03.md b/Semaine_04/Jour_03.md
index bacddf8..ae0607c 100644
--- a/Semaine_04/Jour_03.md
+++ b/Semaine_04/Jour_03.md
@@ -51,4 +51,11 @@ objet.append({"titre": "Alice aux pays", "auteur": "Caroll"})
 
 with open("livres.json", "w") as fichier_json:
     json.dump(objet, fichier_json, indent= 4)
-```
\ No newline at end of file
+```
+
+## Les classes
+
+### Méthodes "Dunder"
+
+- __init__
+- __repr__
\ No newline at end of file
diff --git a/Semaine_04/ProgOrObj/.python-version b/Semaine_04/ProgOrObj/.python-version
new file mode 100644
index 0000000..e4fba21
--- /dev/null
+++ b/Semaine_04/ProgOrObj/.python-version
@@ -0,0 +1 @@
+3.12
diff --git a/Semaine_04/ProgOrObj/README.md b/Semaine_04/ProgOrObj/README.md
new file mode 100644
index 0000000..e69de29
diff --git a/Semaine_04/ProgOrObj/fractions.py b/Semaine_04/ProgOrObj/fractions.py
new file mode 100644
index 0000000..5ce3a67
--- /dev/null
+++ b/Semaine_04/ProgOrObj/fractions.py
@@ -0,0 +1,33 @@
+import fractions
+
+
+class Fraction:
+    numerator = 1
+    denominator = 1
+
+    def __init__(self, numerator, denominator):
+        self.numerator = numerator
+        self.denominator = denominator
+
+    def quotient(self):
+        """[Fraction] Chiffre sur Chiffre"""
+        return self.numerator / self.denominator
+    
+    def plus(self, fraction):
+        """[Fraction] Addition de deux fractions"""
+        num = (self.numerator * self.denominator) + (self.denominator + fraction.numerator)
+        den = self.denominator * fraction.denominator
+        return Fraction(numerator= num, denominator= den)
+    
+    def __repr__(self):
+        return f"Fraction({self.numerator}/{self.denominator})"
+    
+tiers = Fraction(numerator= 1, denominator= 3)
+quart = Fraction(1, 4)
+
+# print(tiers.__dict__)
+# print(quart.__dict__)
+# print(Fraction.__dict__)
+
+fraction = tiers.plus(quart)
+print(fraction)
\ No newline at end of file
diff --git a/Semaine_04/ProgOrObj/fractions_theorie.py b/Semaine_04/ProgOrObj/fractions_theorie.py
new file mode 100644
index 0000000..6d3fb8b
--- /dev/null
+++ b/Semaine_04/ProgOrObj/fractions_theorie.py
@@ -0,0 +1,38 @@
+from pprint import pprint
+
+prenom = "John"  # crée une instance (obj) de type 'str' et l'initialise avec la valeur 'John'
+nom = str('Doe') # crée une instance (obj) de type 'str' et l'initialise avec la valeur 'Doe'
+adress = str()   # crée une instance (obj) de type 'str'
+
+# list, dict, tuple, set, bool => types primitives
+
+class Fraction: # pour se differencier des types primitifs
+    numerateur = 1
+    denominateur = 1
+
+    def quotient(self): # self = tiers /quart
+        return self.numerateur / self.denominateur
+
+tiers = Fraction() # crée une instance (obj) de type 'Fraction'
+tiers.numerateur = 1
+tiers.denominateur = 3
+quart = Fraction()
+quart.numerateur = 1
+quart.denominateur = 4
+
+print(tiers.__dict__)
+print(quart.__dict__)
+
+# Mapping proxy
+# pprint(Fraction.__dict__)
+# pprint(tiers.__dict__)
+# print(tiers.denominateur)
+
+# 2 points de vue: Fraction (classe) et tiers (objet)
+print(Fraction.quotient) # <function Fraction.dire_bonjour at 0x100eec720>
+print(tiers.quotient) # <bound method Fraction.dire_bonjour of <__main__.Fraction object at 0x1028937d0>>
+# <bound method Fraction.dire_bonjour of tiers>
+
+print(tiers.quotient()) # TypeError: Fraction.dire_bonjour() takes 0 positional arguments but 1 was given
+# Les methodes lies aux objets ont besoin de savoir QUI les a appeler
+print(quart.quotient())
\ No newline at end of file
diff --git a/Semaine_04/ProgOrObj/main.py b/Semaine_04/ProgOrObj/main.py
new file mode 100644
index 0000000..e69de29
diff --git a/Semaine_04/ProgOrObj/pyproject.toml b/Semaine_04/ProgOrObj/pyproject.toml
new file mode 100644
index 0000000..f732692
--- /dev/null
+++ b/Semaine_04/ProgOrObj/pyproject.toml
@@ -0,0 +1,7 @@
+[project]
+name = "progorobj"
+version = "0.1.0"
+description = "Add your description here"
+readme = "README.md"
+requires-python = ">=3.12"
+dependencies = []
diff --git a/Semaine_04/ProgOrObj/uv.lock b/Semaine_04/ProgOrObj/uv.lock
new file mode 100644
index 0000000..215c721
--- /dev/null
+++ b/Semaine_04/ProgOrObj/uv.lock
@@ -0,0 +1,8 @@
+version = 1
+revision = 3
+requires-python = ">=3.12"
+
+[[package]]
+name = "progorobj"
+version = "0.1.0"
+source = { virtual = "." }