Stupid question but just want a second opinion.

[mlstrass 5,000+ posts]

Could I use 4 gauge instead? Just shave down the end until it fits? I just have lots of 4 gauge lying around.
Also am I going to have to buy some rca splitters? Or do I just take a set of RCA's and run them from my current monoblock amp's RCA outputs to the RCA inputs on the 4 channel?

4G wire would be fine.

Yes 2 Y splitters to feed all 4ch's of the comp amp. Don't daisychain off the sub amp unless it's getting a full signal and you're using the LPF on the amp.