diff --git a/ex4.py b/ex4.py
index d04bf79..17d488a 100644
--- a/ex4.py
+++ b/ex4.py
@@ -1,15 +1,51 @@
+"""
+Author: Lukas Heiligenbrunner
+Matr.Nr.: K12104785
+Exercise 4
+"""
+
 import numpy as np
 
 
-def ex4(image_array: np.array, offset: (int, int), spacing: (int, int)):
-    mask = np.zeros(shape=image_array.shape, dtype=image_array.dtype)
-    test = list(range(offset[0], mask.shape[1] - 1, spacing[0]))
-    print(test)
-    pass
+def ex4(image_array: np.array, offset: (int, int), spacing: (int, int)) -> (np.array, np.array, np.array):
+    if not isinstance(image_array, np.ndarray):
+        raise TypeError
+
+    if len(image_array.shape) != 3 or image_array.shape[2] != 3:
+        raise NotImplementedError
+
+    if not all([str(x).isnumeric() for x in offset + spacing]):
+        raise ValueError
+
+    if not all([0 <= x <= 32 for x in offset]) or not all([2 <= x <= 8 for x in spacing]):
+        raise ValueError
+
+    src = np.transpose(image_array, (2, 0, 1))
+    mask = np.zeros_like(src)
+
+    for x in range(offset[0], mask.shape[2], spacing[0]):
+        for y in range(offset[1], mask.shape[1], spacing[1]):
+            mask[:, y, x] = 1
+
+    # masking the input
+    masked = src * mask
+
+    # calculate the remaining pixels
+    rest = src * (1 - mask)
+    rest = rest[1 - mask > 0]
+
+    # raise error if known pixels are less than 144
+    if (src.shape[1] * src.shape[2] - rest.size / 3) < 144:
+        raise ValueError
+
+    return masked, mask, rest
 
 
 if __name__ == '__main__':
-    ex4(np.array([[(0, 0, 0), (0, 0, 0), (0, 0, 0), (0, 0, 0)],
-                  [(0, 0, 0), (0, 0, 0), (0, 0, 0), (0, 0, 0)],
-                  [(0, 0, 0), (0, 0, 0), (0, 0, 0), (0, 0, 0)]
-                  ]), offset=(1, 1), spacing=(2, 2))
+    f, d, g = ex4(np.array([[(255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255)],
+                            [(255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255)],
+                            [(255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255)],
+                            [(255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255)],
+                            [(255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255)]
+                            ]), offset=(1, 1), spacing=(2, 2))
+    print(g)