"no matter the credit count the game runs 4 player.
Any suggestions on where to look here?"
Step 1: Get the schematic. Lots of sources for this. Pinball Resource is an easy call
Step 2: Check the player step-up unit. If it's continuously stuck on 4 player, it's not decrementing on start-up. Probably frozen in place from the old grease. Needs to be cleaned and lubed to function properly.