What are the effects of trimming the diameter of a wire down to half its gauge?
If you simply cut strands from the 4 AWG wire, you will have an unpredictable resistance between those strands and the remaining strands. If you don't use all of the strands for the majority remaining length of the full wire size, the voltage drop will be the voltage drop will be somewhere between the voltage drop of 8 AWG and 4 AWG for that length.

Depending on the method of supporting the wire, vibration or just the weight of the heavier wire could put excessive strain on the short length of reduced-size wire.

I would attach ring lugs for 8 AWG on the leads form the slip rings and lugs rated for 4 AWG on that wire. Then, in a junction box, connect each phase of 8 AWG to 4 AWG using short bolts through the ring lugs. Tape up the lugs with appropriate electrical tape.