You'll need to look at the coin unit stepper. It's probably stuck at the 4 player position. I believe it's located in the cabinet. Should be a couple coils one to advance players and another to reset it back to zero. Run each manually by hand and make sure no wires broke off the reset coil lugs.

Hi PW79
I do not have an Target Alpha - I do have an "hard to read copy" of the schematics, I am not familiar with the very late Gottlieb EM pins - I do have a Surf Champ, August 1976 - but it is not a "very late Gottlieb". Do YOU have schematics / manual ?

(((Coin Unit is the Unit for "How many players want to play the next game")))
When starting a game on my Surf Champ the pin lets the "Coin Unit Reset Coil" fire ONCE. I believe to read in my Target Alpha schematics: At start-up the Target-Alpha does step once or twice or 3,4 5? times the RESET-Coil. A thing that supports my theory is in this video: http://www.pinrepair.com/em/index3.htm#axrelayvideo - starting at 1:05 Clay shows a "1977 Jungle Queen" - at 1:23 to 1:30 he manually steps up 5? times the Coin-Unit and DOES SINGLE STEPS to step down the Unit.

Unplug the 110VAC power cord (Safety Reasons), locate the Coin-Unit and manually step it into one direction and then step it into the other direction and finally step it to "somewhere in a middle position" - means "at next start-up": The pin MUST step DOWN - do You see some reaction on the Coin-Unit ? Greetings Rolf

Hi PW79
an add-on to my post-3: In the video You see: Clay FIRST steps the Unit UP (to 2,3,4 players) - THEN he steps it down completely. So want to try to start a game having this Unit stepped-down completely ? What happens ?

IF (if) the pin does faulty step up several times (to 4 players): At schematics-20-E is the "ADD Coin-Unit Coil" - a Make-and-Brake-Switch on PULLED AX-Relay (hard to see in my copy - maybe BX-Relay?) hinders this stepping up.
The AX-Relay is an Interlock-type Relay - it has two Coils mounted, it has two stable positions to be in - may want to try starting a game with "AX-Relay is in this position" and also want to try to start a game with "AX-Relay is in other position" ? General question - when starting a game: Does the AX-Relay move and after some time the relay moves back ? Greetings Rolf

From Mr Pinball
The following information is taken from the November 1976 Gottlieb Target Alpha manual. This may be helpful when debugging a startup problem with this or other Gottlieb 4-player games from the same period.

1. Inserting a coin or pushing the replay button actuates 'S' relay (Start relay).
2. This relay will lock-in through its own switch and a motor 2B switch.
3. 'S' relay starts the motor running.
4. 'AX' relay actuates and the 'coin' unit subtracts through switches on 'AX' and 'S' relays and switches on motor 4C and motor 1A. 'Q' relays de-energizes when 'AX' relay operates.
5. When 'AX' relay actuates, the reset relays 'Z1' and 'Z2' are energized in sequence by rivets on the 'Player' unit, through switches on 'AX' relay.
5a. Motor 1A steps the 'Player' unit, through switches on 'AX' relay and 'P5B', to the 20th position energizing 'Z1'.
5b. When 'Z1' is energized the 1st and 2nd player score units reset to zero through switches on motor 1A, motor 4A and 'Z1'. When all score units of the 1st and 2nd player are at zero the player unit steps to the 21st position through a normally closed switch on 'Z2' and normally closed switches on all score units of the 1st two players, actuating 'Z2' relay. The 3rd and 4th player score units now reset through switches on motor 1A, motor 4A, and 'Z2'. The player unit continues to its zero position through the closed switches on 'Z1', 'Z2', 'P5B' and motor 1A until 'P5B' opens.
6. When 'P5A' on the player unit closes, 'AX' and 'BX' relays reset through motor 4C and switches on 'DX', and 'R' relays. The reset cycle is now complete.
7. Inserting additional coins or pushing the replay button will step the 'coin' unit through switches on 'DX' and 'S' relays and a switch on motor 4C. The replay button circuit opens when 'coin' unit is on 4th position.
8. Place the ball in the out hole. The ball return switch closes and completes the circuit to 'O' relay through the normally closed switches of 'DX', 15th position 'Bonus' unit, motor 1C, motor 2B, 'Q', 'AX', and 'S' relays. 'O' relay locks-in. Through switches on motor 4C, 'BX', and 'O' the ball return coil actuates kicking the ball onto the runway.
9. The remaining balls that enter the outhole are kicked across the trough switch which pulls in 'P' relay. 'P' relay runs the motor. Switches on 'E', 'DX', motor 2C, 'P', 'BX', and motor 1A advances the 'Player' unit the required number of steps determined by the 'Player' unit switches and the 'Coin' unit.
10. The 3-5 ball adjustment changes the number of balls played per game. The third or fifth ball leaving the playfield will trip the 'last ball' relay 'BX' through switches on 'E', 'DX', rivets on the player unit disc, and 'O' relay.

In addition to the video by Clay as rolf_martin_062 links to , you will also see a Target alpha coin unit photos in this section below

▬ http://www.pinrepair.com/em/index2.htm#steps ▬

as well as its state in player 1 and player 4... This should also help get you on the right track...

Moved to EM Tech, better place for this thread.

As schudel5 said in post #2, the 4-player problem is most likely due to the coin unit being stuck. The coin unit is the large stepper mechanism mounted on the bottom board, in the front right corner of the board just behind the chime unit.

For your newest machine issue, it sounds like it started after you manually checked some relays. This happens sometimes. It would be a good idea to go back and examine those relays to see if any switches on a relay are now out of whack, or perhaps a relay has gotten stuck in some way.

If you've never worked on an EM before, it could be helpful to read up on it here:
http://www.pinrepair.com/em/index.htm

- TimMe