Monitor question -> 4k and 1080p

I have no interest in 4k at this time, just something picking my brain. I am well aware of the requirements to run 4k well on "ultra" settings on any game.

My question, is what does "medium" or "low" look like in context to 1080p? Would "medium" on 4k be equivalent to "ultra" on 1080p? (speaking purely visual, not computer requirements). Just thinking with higher pixel density, surely these are over lapping at some point right?

Only as it pertains to pixels-per-object. Changing to medium & low changes not just the detail, but the things you see on screen. The actual textures change to show less detail. So while there would be tons of pixels with 4K low, it wouldn't be showing you everything you could see on a 1080p monitor running ultra.

I can't think of specific examples right now, but run a game on ultra for a while, then switch to low and watch the changes. Alternatively, go to a scene with a lot of detail on ultra, take a screenshot, switch to low and then take another & compare.

Interesting. I was thinking the pixel density would cover the lack of detail between the 4k and 1080p.

How about apples to apples then, medium on 1080p looks worse than medium on 4k?

In theory, yes...but unless you are talking larger monitors (28"+ maybe?...not sure exactly where the line of obvious difference is), it would be difficult to tell while gaming.

I have a 28" Hanns G HH281 (1080p monitor), 30" Apple Cinema Display (2650 x 1600), and a 28" Samsung UHD (4K) monitor. I can max out settings on all of my games at 1080p, max out or close to on the Apple, and low-medium settings at 4K with a GTX 980. I can say that the visual appeal of 4K can justify the lower settings. I use the GeForce Experience settings and most of my games are set for 2650 x 1440 with high settings (e.g. Far Cry 4, Tomb Raider 2013, The Witcher 2.), and a few has it set to 4k (3840 x 2160) at low-high settings (e.g. Titan Fall, Batman: AO, Crysis 3, Skyrim.)