A 2017 iMac 4k has two memory slots, which are on the back side of the logic board. You have to pull out the logic board to do an upgrade. Not a simple upgrade...
Yeah I was looking at the ifixit steps, seems involved but this thing is just unusable as it is. I'm going to put an ssd I have on it and I figured might as well upgrade the ram at that point since I don't see me taking it apart again in the future.