Bet the flop because there are 4 people in the hand and someone will come along with garbage for "cheap". Plenty of people limp PP and Ax pre-flop and will have an overpair/top pair/middle pair they like (not to mention overcards).

Bet the turn because no one's buying you're on trip 4s, and if you're lucky enough to have hit someone with something like A4 you'll get all kinds of action. Plus, you've got the bottom boat on a paired board and you don't want anyone getting to their two-outer for free.

Call the river because the doofus with 76 and the idiot slow-playing his trip 4s on the turn both just drew out on you, and it's gonna suck to get bluff-raised off the best hand if no one has a better boat.