It's Hopper-specific, the improvements are closely tied to Hopper features like warp groups and TMA. For 4090s, you might get a speedup by using the Triton implementation of FP8 attention: https://triton-lang.org/main/getting-started/tutorials/06-fu...
The original flash attention (v1?) took like a year to get added to llama.cpp and only provides single digit percent VRAM savings for typical context lengths and practically no speed boost. Still nice to have, but man was this thing overhyped. I doubt v3 will do more than marginally better on the RTX 5000 series.