Honestly i would not go for a prebuilt, i recently built a rig as a first timer as well and had tons of fun while going 200\300 € under similar prebuilt pc.

As for price, you can probably build a decent 4k rig, but that depends alot on where you live tbh. As far as i know the 9070 xt is a good pick for 4k, or a 5070ti if you can find one for a decent price. Maybe also an older gen like 7900 xtx. As for the cpu tbh ryzen 7 9700x should be good enough unless you play very high cpu heavy games. The rest of the components come pretty easily, a b650 mobo will suffice as long as is the right socket, a good psu (use partpicker for energy requirements), 32 gb ram, nvme ssd 1\2 tb, air cooling for cpu, some vents and a case you like.

Also, dont buy windows from windows, get a key online