So after looking up benchmarks I'm probably going to go with M4max unless Apple boosts the clocks on the M3ultra a lot. Between the core-for-core performance increase and the fact that a 4k budget cap on my part means I'm comparing an unlocked max to a binned ultra, there's only a 20% difference in GPU power. I calculated that twice, using
this guide and a second time using blenderscore (multiplying the binned m3 max score by 1.9 since that seems consistent across m1 and m2 ultras) and got about the same result.
I'll still wait for the benchmarks but I'm pretty sure of what I'm getting.