I game at 3440x1440 ultrawide, and upgraded from a 3090 to 4090. 4090 is significantly faster and smoother. And DLSS3 frame interpolation is no joke- in Hogwarts Legacy with every setting cranked up and max ray tracing, turning on DLSS3 jumped me from around 80fps with noticeable 1% low stutters and now pegs it at the 144fps limiter I set in game. Smooth as butter.
Also I mess around with some AI things like stable diffusion, and it's much faster for that as well. As much as I hate the term "future proof", the 4090 is more worth it in that regards, IMO.