I have a 20M2U, and from what I can tell the M4U differs only in the resolution of the tube (horizontally; TV lines) and the 20M4U's separate H and V focus knobs. If I come across a Midway PCB, I'll see if my 20M2 accepts it.
You might have luck adjusting the V hold in both the user menu and service menus.
Any instructions for him to access those menus or are they on a remote?
I know you have probably already tried this, but I had a rolling screen/sync issue with my picky pvm and Raiden II board. What I need to do is boot the game up first, then turn on the tv and if that doesnt work, just turn the tv off and then on again while the game is running. This forces it to finally sync.