If you wire in parallel, you're going to need 8 batteries. You have to wire them in series.

What I do is unsolder the red power wire from the pcb and run it to the top. I use that to run to the negative of the extra battery holder. Then run the red wire back and solder it to where you took the original off. I just leave the batteries on the top of the chasis. Once the body is on, it seems to stay pretty snug.