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i have a 4 ch amp that i want to power all four of my speakers. The amp has rca slots for ch 1 and 2, ch 3 and 4, and it says pre out 1-3 and 2-4. So there are three rca slots on the amp. There are 2 rca slots on the HU. one for rear and one for front. Do i need two rca cables, one for the 1 & 2, and one for the 3 & 4 rears to make the amp power all four channels?

Yes.

you need 4 RCA lines. two cable pairs.
one per channel from the head unit