Is the LED on solid? If it doesn't get the little first 'flicker' (not flash) then have you tried to short U9 pins 39 and 40 briefly to reset the MPU and see if the issue is in the reset section?

Are you saying that you only have 4v on the MPU?

Rebuild the male AND female connectors (at the same time), because that is low.

(I'm assuming you already rebuilt the the driver board capacitor)

https://pinside.com/pinball/forum/topic/vids-guide-quick-bally-driver-board-repair-bulletproofing

Sounds like you're getting sloppy/low power to your MPU according to this:
http://techniek.flipperwinkel.nl/ballyss/rep/index1.htm (search for TP1 in your browser on that page)

Fortunately on the same site is an extremely in depth guide (linked below) on how to diagnose Bally MPUs and I would start from the very top and work your way to the bottom and report back assuming it doesn't get fixed along the way.
http://techniek.flipperwinkel.nl/ballyss/rep/index2.htm

If using a standard computer (desktop) power supply as bench power you will most likely need to jumper the green wire on the 20/24 pin connector to any black wire on the same connector to actually fully turn on the power supply. More info on that is here:
http://www.instructables.com/id/How-to-power-up-an-ATX-Power-Supply-without-a-PC/

Update: I should note that once you get the MPU working on the bench power you'll still most likely have to tackle the driver board (capacitors) and possibly rectifier board since they still will most likely be putting out bad power. The same website has another detailed breakdown on how to proceed and even has some nice bulletproofing steps.
http://techniek.flipperwinkel.nl/ballyss/rep/index1.htm