This is odd. To clarify, you can make the “1 to 4 Can Play”, display on the back glass any of the four possibilities 1 - 2 - 3 or 4, but still plays a four player game? Why this is odd! The unit that lights the 1 to 4 can play lights is the same unit that will control the number of actual players. Have a look at the fingers of the Coin unit; see if there is something odd there first.

Coin Unit, better name would have been "Number of Players" unit.

And any modifications, New wire or solder.... Player Up unit too, same sort of thing...

If the Coin unit and Player up units both look good, check contacts of the Jones plug. Which Jones plug is marked up on the schematic.

Looking a little closer at the schematic and your OP.

You can select the number of players but it always plays a 4 player game.

The coin unit manages the number of player for the game. The player unit is managing which player is playing. If you look at the schematic snippet that Howard has posted (Better Quality than mine) and Howard has drawn the path for a single player game.

Once the ball drains, the score motor starts as we have closed the Outhole SW and energised the Outhole RE. SW 3D will close as the scr motor rotates and this will fire the Ball Count Stepup coil advancing the ball count unit.(Single Player game)

If you play a 2 or 3 player game, the above will not happen as there is no connection from the SW in the outhole RE to the ball count step up coil until the Player Up number = the coin unit (Number of players). In this case when the ball drain, SW 4b will close during the scr motor cycle and fire the player up step up coil advancing the Player up unit.

If you play a four player game, you don’t actually use the Coin unit as describe above, see schematic below. Here I can see that it is possible to have a 1,2 3 or 4 player game and always having 4 players playing if there is an issue with how the Coin unit and Player up unit interact.

So what would make this happen? From the other posts, It appears that the Coin unit works, it also appears that the Player up unit works too. But not together. Not sure how the Ball Index RE is working here, It's purpose is to NOT advance the player or Ball count units if no points are scored for a ball when it drains.

I guess the issue is most likely at the Player up unit or the connection between them (Johns Plugs).

I have marked up the wire locations on the jones plugs with a red box.

Help (resized).jpg

help1 (resized).jpg

PS, Double check the Wire numbers for the Jones plug, my schematic is a bit hard to read. Think I have them right, after 3 goes.....