It looks to me like Tank number 2 has the labels reversed. I'm not sure I agree that you have to have "two tanks in parallel". You can run four tanks in series no problem. In my megabar, I have one CG tank that is connected from the vent to two tanks in parallel and then out through a T to the UAT.

So from the UAT, I go to a T and then to two main tanks, then they go to a T into the CG tank, then out through the vent. To add another tank on top of the CG tank, in series would be no problem. At that point it becomes an issue of pump head, and loss through your system.

My clue to the reversed nomenclature is that the last tank is full, and then drains at the angle you have it into the next tank.

FWIW, I can not tell where the overflow line from Tank 2 goes, but I assume it goes into the one of the saddle tanks, or a T.