Nvidia RTX 4090 is allegedly coming with the same memory configuration as the RTX 3090 Ti. According to prominent leaker kopite7kimi, the RTX 4090 will be based on the AD102 GPU, featuring GDDR6X memory. The leaker has also fleshed out his previous leak about the RTX 4070's memory spec.
https://www.notebookcheck.net/Leak-points-to-Nvidia-GeForce-RTX-4090-allegedly-coming-with-24-GB-of-ultra-fast-GDDR6X-memory.619086.0.html
I'd like to see AMD have both 16 and 32 GB options for the top card in the lineup (or allow AIBs to go to 32 GB), just for the lulz.