That big ugly hunk of iron that Ford used for an intake is a bitch to get on the engine without dislodging or creasing an intake gasket. Number 4 is an end cylinder on a Ford, and there's a water passage next to the end intake ports, so it could be leaking there. The gaskets have a channel built in to them to carry any seepage to the end of the head and out on the ground, but this often gets filled with gasket goo and doesn't do the job. Pull the manifold, clean everything good, and use thick composite gaskets, DRY, no gorilla snot anywhere except on the end seals. Lower the manifold straight down, gently(this might require two people, the damn thing weighs a ton) so you don't have to wiggle it around much to get the bolts in. Torque it starting at the center and working out to the ends. Should solve the problem, unless a head gasket just decided to pack it in while you had the intake off, which is unlikely.