The problem I usually encounter with the 4 LEDs on is the clock circuitry on the CPU board has failed. Especially the two 7474 socket chips near the bottom of the board by the crystal and next to the battery holder. If you have acid damage these 2 sockets probably have broken pins and need to be replaced. If they check out ok, trace the clock pulse from the 12mhz to the 6mhz and to the 4mhz to see if all is ok, most likely one of these 7474 or socket is bad.
Also try this, with the game on, reach through the coin door from the front of the game and wiggle and push on the ROM ribbon cable and watch the monitor for any change. If the ribbon cable or connection to the CPU board is bad this can also cause the 4 LED error code.