4k TV Docked Question: RP6 v.s RP5 ?

I was lucky enough to get a new 4k TV arriving soon but I am concerned about the docked experience on my old RP5. Doesnt the RP5 only output 1080p max? Will the TV or the device have to do some upscaling, will it work at all, if it does will it look bad?

I was thinking of upgrading to a RP6 or equivalent Ayn device for several reasons, one of them being that they can output 4k to the new 4k TV. Is that a valid plus or will the RP5 and some sort of upscaling be good enough for playing my games (Gamcube and PS2)?

The 4k TV is the LG C4 if that matters, or is there a better 4k TV to pair with the RP5 (or RP6 device) around that price range?

Thanks, sorry if i am misunderstanding something basic!