Answer: 16
Reasoning:
There are 2 outcomes if you flip only one coin (H or T).
There are 2^2 = 2*2 = 4 outcomes if you flip two coins (HH, HT, TH, TT).
There are 2^3 = 2*2*2 = 8 outcomes if you flip three coins.
There are 2^4 = 2*2*2*2 = 16 outcomes if you flip four coins.