Johnny is right. When a call is placed SU or SD is energized. That is the beginning of you run sequence. SU picks UA which pick U (yes there is more too it, I'm paraphrasing). And as he has asked, it would be helpful to know if you have prints.

Dover units usually have the job # (E??????) and the print #s listed on the controller. If we knew the print #s we could look up information quicker and with more ease.
Here is what I'm woundering. Both cars keep returning to the 1st floor. Is the 1st floor the home landing? Are these units setup for home landing? Have you checked the normally open contacts that when closed hold in the 2 & 4 car calls?

I'm heading in this direction from your comment that there is no demand.

Let us know how you are making out...