So, are all the numbers in this sequence supposed to have prime numbers before and after them??? I feel kind of dumb for having to ask, but that is what I deduce from these hints and what not. If that is the case, then 4 does fit in, and you would pretty much have to look at a prime number chart to see the series. 78 also works as a solution, but for a different reason, and the pattern would have to jump around and make little sense, as with 72 as an answer with a sequential pattern. Mike mentioned something about SAT’s. I could see this being a question on one of those tests, of course with only one correct answer and only one correct explanation.