You can convert a slash into a traxxas rally. I believe you would need rally spec arms, turnbuckles, wheels/tires, body, body mounts, bumpers, ect. Don't know the 4x4 platform well yet. But I seen lots of people convert slash 4x4s into Fiesta Rally's.
I think you’d need new body mounts & bumpers at least
I know that before the LCG Slash 4X4 platform came out, there was the Rally so they’re very similar