4GB was enough for the vast majority of games but there are also PS4 ports that can use 10GB (final fantasy for example) or close to it (RE2 remake) at 4K.
At 8K VRAM requirements are even higher, and just look what happens when 2080ti will run out of VRAM
It's a slide show.
Nearly 20GB VRAM usage in certain games and these are just PS4 ports.
Next gen games will be build with SDD streaming in mind, so developers will be able to stream inasane amount of data even if next gen consoles will launch with just 16GB (13GB for games and 3GB for OS).
On PC market you can buy really fast SDD drives, but these arnt build with games in mind, so data decompression speed will bottleneck SDD speed drastically (XSX and PS5 will use HW decompression). Without equally good SDD, developers will be forced to increase memory requirements on PC. Mark my words, PS5/XSX ports will use 20GB (if not more).
The SSD isn't going to save the memory bandwidth problem. Like, at all.
If you look at the entire pipeline memory stage from CPU RAM (resident) to GPU registers (in like a Volta architecture), here are the different bandwidth ranges at each stage
CPU RAM ~ 150GB/s
PCI-E (SSD controller bus) ~ 16GB/s -- OR -- using NVlink (~50GB/s)
GPU RAM (fetch/store) ~ 900GB/s
L2 cache (fetch/store) ~ 2000GB/s
L1 cache (fetch/store) ~ 2000GB/s
GPU registers/shared memory pool ~ 20000GB/s
So the bottomline is to still try to get as much data into RAM (either CPU or GPU) as possible. I'd rather have significantly more VRAM and a regular SSD than to have less VRAM and a faster SSD. Next-gen consoles will be starved for VRAM/RAM since they use a shared memory model.