You MAY have to pull the head (little more testing needed yet). I'd get the book double check the timing on the engine (cam/crank). 3 at 90 and one at 78 difference of 12 psi doesn't sound that bad but since 3 were consistent and one wasn't and they were all a bit low (120 is closer to normal usually) I'm thinking a cylinder leak down test is your next step. Have you done one before? It should give you a definite direction as to what needs attention base engine or head wise.