To find probability that for the 7th toss head appears exactly 4 times. We have find that past 6 run of tosses.
Past 6 tosses, What happen we don't know. But according to 7th toss, we will find that head appeared for 3 times.
Any coin probability of a head=$1/2$ and probability of a tail= $1/2$
If we treat tossing of head as success, then this leads to a case of binomial distribution.
According to binomial distribution, the probability that in 6 trials we get 3 success is:
$\binom{6}{3}*(1/2)^{3} *(1/2)^{3}= 5/16$
7th toss, The probability of obtaining a head=$1/2$
In given question is not mention that what happens after 7 tosses.
so Probability for that for 7th toss head appears exactly 4 times is
=$(5/16)*(1/2)$
=$5/32$