Regarding the cost of 4090 GPUs, I used to work in a 3D software company and our graphics expert Ray was convinced the half life of a GPU was 6 months.
Faster cards come along making the old cards less competitive and unwanted. Ray always got the latest greatest in his test PC!
So yes wait until the 5000 series is released. There maybe deals on the 4000 series.
PS His nickname was Ray Tracing 