Right off the bat, I will let you know the game is not optimized to run at 4k at all. However, I will say it looks gorgeous and crisp and great for screenshots. I switch between 4k and QHD, most commonly known as "2k". There is a significant dip in performance when going into battle at 4k, so I just play at 2k. Here are my PC specs:
https://pcpartpicker.com/user/Zombwrlck/saved/#view=p4bJyc
Aside from my CPU (thanks to scalpers I couldn't get what I actually wanted, so settled with the Ryzen 9 3950x), I don't think my computer needs to be upgraded anymore unless I was going to get 120gb of RAM, which is absolutely unnecessary for anything I'm doing. Now there are new GPU lines but I still don't feel the need to upgrade. The 6900xt is a beast that can handles anything I throw at it even in 4k... except BnS. 🤣
If you are going to upgrade your computer, do not do it with this game in mind. Because it will always disappoint no matter what. My computer is the way it is because I do a lot of casual 3D work and I'm always playing newer games as well.
I have several screenshots of recent floor clears our team did, so you can see what can be expected. I'm playing the game at 2k res.
These are stream vods, so the quality isn't as good as if I had been capturing directly or taken an actual in-game screenshot but you can see the framerate. My in-game settings are all at 5 except for other players effects, which I will keep at 1 or 3 if the floor we are doing has a buff that spawns on other players that needs to be seen at all times. The settings at 1 hides that effect, so you are forced to play with settings set to at least 3. It is impossible to play the game with other players effects set to 5 even with my computer. Having shadows on in certain dungeons is very detrimental to performance as well. An example of this is the Aerodrome dummies room, where the game just runs horrendously if you have shadows on but then settles down once you get out of that room.
I'll use the newest dungeon, Grim Nexus, for my examples since this is what's currently relevant to the game and what players are actually running.
The game always runs really bad during trash mob pulls.
An example in Nexus here, you can get some 20 FPS when all mobs are getting hit at once. Game actually COMPLETELY froze here as well. My other teammates experienced the same game freeze, too.
It isn't all bad. During burst, I'm getting around 40 FPS. You can see how much is going on. By no means what it should be given my specs but it's playable. ¯\_(ツ)_/¯
During actual combat and even with all these effects on the screen during phases, I'm still getting 40-50 FPS. It's smooth to play in. During no phases, I get as high as 55, so this is basically the best you are going to get in battle imo. Also to keep in mind is that I'm streaming during this as well with OBS and that also is hogging my resources but I don't feel any difference between when I stream and play normally.
I'm sure I can get better performance by playing in lower resolution, too. But it's annoying to keep switching resolution between games that I play so I just leave it as is plus you asked about 4k. I don't have 4k footage but it does run significantly worse for sure when I did use to play at that resolution. Expect a 10-20 FPS loss or more in 4k. Game just isn't made to run at that resolution. I will say the game looks great when you can actually play with almost everything maxed and I think it's good eye candy for viewers when streaming compared to other MMOs.
I'm sure this may be disappointing to you but just wanted to give an honest testimonial about the performance of the game with high specs.