If there are 15 people in the house, the number of people eating pizza is 3/5*15 = <<3/5*15=9>>9 people.
If each house member who eats pizza took four pieces to eat, the number of pieces taken is 4*9 = <<4*9=36>>36 pieces.
Since the total number of pieces was 50, the number of pieces not taken is 50-36 = <<50-36=14>>14 pieces.
#### 14