The probability of getting exactly one head is equal to the probability of getting exactly one tail, or exactly three heads. These are mutually exclusive events.
The probability of getting one head is given four ways out of 24=16 possibilities, so 1/4.
1/4 + 1/4 = 1/2
n-choose-r times pr(1-p)n-r
Use it once with n=4, r=1, and once with n=4, r=3. In each case, p = .5
Then add the two results.