Response from neighbor:

2nd floor, is square, does not contain digit 4, is odd.

Square numbers between 21 and 50: 25, 36, 49

Does not contain 4 leaves 25 and 36

Is odd only leaves 25.

Person in flat 25 says that neighbor lies about everything.

So must be 1st floor, not square, contain 4, and is even;

Numbers 1-20 eliminate odd numbers

2,4,6,8,10,12,14,16,18,20

Eliminate squares:

2,6,8,10,12,14,20

Only 14 contains digit 4.

Flat # 14 correct