Presumably, the coin tosses are independent of one another, so
[tex]P(\text{exactly 4 heads}\mid\text{first flip is tails})=P(\text{exactly 4 heads})=\dfrac1{2^4}=\dfrac1{16}[/tex]
But if the coin flips are not independent, then we can't really say anything about this probability without any more information about how the flips are related...
