You could have posted a reply with these pictures in the other thread ... limit is by post, not thread.
Any hoo ...
I have been avoiding jumping into this but it has become painful to watch and I need some NoobKarma(tm).
The first thing to remember is that JAMMA only handles two players. So, the JAMMA harness thatyou are swapping only has connections for two players (this will be why switching the harness does not give you four players on other boards).
The controls for players 3&4 are connected to the Wrestlefest PCB using two molex connectors (they connect at C2 and C3 right beside the JAMMA edge connector). These connectors are sometimes referred to as a kick harness.
Wrestlefest manual can be found here:
http://happycampers.to/arcade/manuals/Information about the connections is on pages 9 and 10.
You need to make the same connections when you swap boards, BUT the wiring pinouts are likely different beween boards so you will need an adaptor.
Having just read your other threads, it doesn't sound like you want to build anything, so your only real choice is to stick to true JAMMA (2 players, 3buttons each) games if you want to swap boards without dealing with wiring and adapters.
If you do want to make adapters, then your next step is to find the pinouts for the extra connections on the "other" board you want to swap out. Compare it to the pictures on page 10 of the Wrestlefest manual. From there, you need to obtain the appropriate molex adapters and run wire from point to point (e.g. Player 3 Right goes from Pin 1 on the Wrestlefest pinout to Pin 4 on TMNT:Turtles in Time).
If this sounds confusing, grab the manuals and look at them side-by-side.
Good luck!
Cheers.