Hi,
There is not strictly speaking, connector dedicated to the bottom displays (3 and 4), however there is a common part.
The displays 3 and 4 use the same group for controlling the segments, which is the
group B. Just look at the diagram to make sure:
2019-07-29_15h33_06 (resized).png
This group circulates (like the others A and C) by the big connector
A1J2.
A garbage display on scores 3 and 4 can actually come from bad contacts at A1J2.
But it can also be an issue on the CPU board (A1).
If A1J2 is safe, then most probably the CPU is defective. Chips involved in the segments group B are
Z20(74175 - latches) and
Z21(7448 - decoder).
Sometimes the failure also comes from the sockets when the board has already been serviced.
A good practice is to use socket when replacing toasted chips. As you can see on the picture below, Z20 and Z21 have been socketed (high quality, tulip).
It can happen then that the IC are badly inserted on the socket, or that they are of poor quality and that contacts are bad.
If so, try to take out the IC and re-insert them (take care of the orientation).
