Simple coin tossing problem?

[PrayingMantis]

This is a simple problem I thought of. Any help will be appriciated, because I'm a bit lost in it...

My friend has tossed a coin 4 times, and written down the results.

Now he tells me: "In this sequence, I got three heads, and one other result, which is either heads or tails. Guess what it was. I'll give you 100$ if you guess it right!"

Well, if I'm treating this in a non-conditional manner, the probability is of course 0.5 for each. So it makes no difference if I say heads or tails.

However, I know that for this 4 tosses sequence, the probability that there were only three heads, is 4:1 to the prob. there were four (i.e, there are 4 ways to get [3 heads, 1 tails], but only 1 to get [4 heads]). So it also makes some sense to assume that the answer is 0.8 for tails, 0.2 for heads. Therefore, I better say tails.

How should I solve this one?