RPi4 2GB has been adequate for my needs, all of my MC devices are 2GB. 4GB for $10 more gives you a lot more headroom, if you're on the fence then I would spend the extra money.
But even at 2GB, Kodi sits under 1GB (usually around 800MB with a very large buffer cache tunable). A fresh start up of Kodi is less than 350MB of RAM used, but things go up as icons are cached, and then the playback buffers add in as you play content.
I do occasionally playback 4K HDR movies, no issues, but most content I have is 1080p. I don't use SSDs, all of my media is on my NAS. I just boot from an SD card.
Now that I am re-reading, I seem to have missed the question, it's not about using it as a MC device, but as a NAS. I have no experience with that, but I can't imagine it being great, really depends on how demanding the clients are.
By the way, what I would suggest is looking at the RPi5, and maybe check out some of the performance reviews / NAS testing from folks like Jeff Geerling. RPi4, in my opinion is very much built to a price point, and some of that sacrifice is I/O bandwidth. RPi5 specifically has a custom chip to address this shortcoming, and adds a lot more bandwidth to USB3.0 and even a PCIe bus for NVMe's.