Anyone know if the M4 GPU support has improved? I might not use Asahi "today" but I'm sure the day will come when my M4 laptop will be deemed "vintage" despite being perfectly fine hardware, and on that day I am most likely to flip over. Hopefully by then the GPU support is much richer.

There is no support for M3/M4/M5 GPUs, period. Asahi Linux is only installable on M1 and M2 Macs.