This question can be answered using the binomial distribution. The probability of 4 successes in 6 tries is
C(6, 4) * p^4 * q^2
Here, C(6, 4) is the number of ways of chosing 4 objects (here, the flips that showed heads) out of 6 tries,
p is the probability of heads (here, it's 1/2, since the coin is fair) and q is the probability of tails (again, 1/2 for a fair coin).