Answer:
Because the answer would be 16 which is 10000 in binary, and the 1 drops off because there are only 4 bits for the answer.
Explanation:
All calculations will be modulo 2^bits.
So 4 bit storage: calculations modulo 2^4=16
16 modulo 16 is 0.
17 modulo 16 is 1. etc., it wraps around after 15.
