Overdoing it just means a little more effort to solder.
4AWG is probably fine, but there is so little down-side to using 2AWG or even 0AWG that I'd suggest just finding the largest you can.
You'll probably end up making that cable; I don't think anyone sells one long enough. It has to go all the way from the starter relay (or battery) under the oil pan to the furthest part of the engine.
Like all RBV starters, this one grounds through the transmission and engine block. This is probably the limiting factor for current draw.
I'd suggest this is a good time to work over the ground-side battery cable as well.