Originally posted by Lecherous Lychee 🐸:

Alternatively, you can download a 4K mod, but you need a pretty hefty machine to run it apparently.

https://www.nexusmods.com/valheim/mods/1030

Try it out, it doesn't use more GPU or CPU just video ram, so if you have a decent amount you will have the HD textures.
I will add I did try it and while it was good, I prefer the default look myself.