I have about $4000, what's the best setup to get?

A user with a budget of around $4000, potentially stretching to $4200, is seeking advice on building a PC setup. The primary goal is to run Qwen 3.8-Flash-Next with decent performance. They have existing NVMEs and DDR4/DDR5 UDIMMs, and possess either an Intel 14700K or Ryzen 9700x CPU, meaning they might only need a motherboard and CPU if a new build is necessary. The budget is a "use it or lose it" work allocation.