The 4090 isn't obsolete; the 5090 just raised the ceiling. If your workload fits in 24 GB the performance-per-dollar argument is still strong.
Availability, checked 2026-09-24: the RTX 4090 Founders Edition launched at a $1,599 MSRP and is no longer sold new. Trade reports in October 2024 said NVIDIA had ended RTX 4090 production, and NVIDIA's product page now offers no buying options. The Newegg link this page carried returns a not-found page and was removed.