There are two possibilities for each flip (Heads or Tails). You multiply those together to get the total number of unique sequences.
Here’s an example for 2 flips: HH - HT - TT - TH. (That’s 2*2, or 2^2)
You want to know how many distinct sequences of heads and tails occur if a coin is flipped 4 times.
That would be 2^4 so 16.
When you toss a coin once, there are two possible outcomes: heads and tails. Tossing a coin four times, on the face of it, appears to yield 2x2x2x2 outcomes, or in other words, sixteen.
I hope that was corect and helpful, also sorry it took me a while.
+256 
Good job!