diff --git a/Week04/decorators_sefa_sagirli.py b/Week04/decorators_sefa_sagirli.py
new file mode 100644
index 00000000..47db1c29
--- /dev/null
+++ b/Week04/decorators_sefa_sagirli.py
@@ -0,0 +1,32 @@
+import time
+import tracemalloc
+
+
+def performance(func):
+    """
+    Measure the performance of the decorated function.
+
+    The decorator keeps three statistics as its own attributes:
+    counter (number of calls), total_time (in seconds) and
+    total_mem (peak memory in bytes used by the calls).
+    """
+    def wrapper(*args, **kwargs):
+        tracemalloc.start()
+        start_time = time.perf_counter()
+        try:
+            result = func(*args, **kwargs)
+        finally:
+            elapsed = time.perf_counter() - start_time
+            _, peak = tracemalloc.get_traced_memory()
+            tracemalloc.stop()
+            performance.counter += 1
+            performance.total_time += elapsed
+            performance.total_mem += peak
+        return result
+
+    return wrapper
+
+
+performance.counter = 0
+performance.total_time = 0
+performance.total_mem = 0
diff --git a/Week04/functions_sefa_sagirli.py b/Week04/functions_sefa_sagirli.py
new file mode 100644
index 00000000..59648e6c
--- /dev/null
+++ b/Week04/functions_sefa_sagirli.py
@@ -0,0 +1,41 @@
+custom_power = lambda x=0, /, e=1: x ** e  # noqa: E731
+
+
+def custom_equation(x: int = 0, y: int = 0, /, a: int = 1, b: int = 1, *, c: int = 1) -> float:
+    """
+    Calculate the result of the equation (x**a + y**b) / c.
+
+    :param x: The first base, positional-only, defaults to 0
+    :type x: int
+    :param y: The second base, positional-only, defaults to 0
+    :type y: int
+    :param a: The exponent of x, defaults to 1
+    :type a: int
+    :param b: The exponent of y, defaults to 1
+    :type b: int
+    :param c: The divisor, keyword-only, defaults to 1
+    :type c: int
+    :raises TypeError: If any of the parameters is not an integer
+    :return: The result of (x**a + y**b) / c
+    :rtype: float
+    """
+    for name, value in (("x", x), ("y", y), ("a", a), ("b", b), ("c", c)):
+        if not isinstance(value, int):
+            raise TypeError(f"{name} must be an integer")
+    return (x ** a + y ** b) / c
+
+
+def fn_w_counter() -> (int, dict[str, int]):
+    """
+    Count the total number of calls and the number of calls per caller.
+
+    :return: The total number of calls and a dictionary of callers with their call counts
+    :rtype: tuple[int, dict[str, int]]
+    """
+    if not hasattr(fn_w_counter, "total_calls"):
+        fn_w_counter.total_calls = 0
+        fn_w_counter.callers = {}
+    caller = __name__
+    fn_w_counter.total_calls += 1
+    fn_w_counter.callers[caller] = fn_w_counter.callers.get(caller, 0) + 1
+    return fn_w_counter.total_calls, fn_w_counter.callers