Can I run Rax-4.5 on an RTX 4090?

Yes

Its published weights (4.55 GB) fit in the 21.6 GB an RTX 4090 can use, with room for ≈ 256K tokens of context.

hf download raxcore-dev/Rax-4.5

Files and context on RTX 4090

Each file of Rax-4.5 (and of GGUF versions the site has analyzed), and how much context fits next to it, with the context cache in 16-bit or 8-bit.

Rax-4.5 on other hardware

Popular models that run on RTX 4090

Assumes 90% of the card's memory is usable and at least 4K tokens of context for one conversation, with the context cache in 16-bit unless noted. Fits leaves at least 20% of that memory for context; Tight leaves less.