Can I run Qwen3.5-4B on an RTX 5090?

Yes

Its published weights (9.32 GB) fit in the 28.8 GB an RTX 5090 can use, with room for ≈ 256K tokens of context.

hf download Qwen/Qwen3.5-4B

Files and context on RTX 5090

Each file of Qwen3.5-4B (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.

Qwen3.5-4B on other hardware

Popular models that run on RTX 5090

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.