I've had both 2k and 4k Gsync gaming monitors for 2 years now, the 4k is a 28" Acer, the 2ks are 27 and 34" Asus ROGs.
I still play at 1080p, for me it isn't so much about the dot size, although that's part of it, it's more about the appearance and size of the targets while fighting - they are just harder to hit. I remember back at a con when Drex famously switched his resolution from the common 1024 most were using back then to 800x600, and his reason was the same - "it's easier to hit targets". I find the same thing happening when going from 4k, and even 1440p, to 1080p. While the game looks incredible at the higher resolutions, it doesn't play as competitively, at least for me.
Just picked up the first 144hz Gsync 4k monitor, it's 27" unfortunately - I'm hoping Asus figures out how to make 144hz work with their 34/35" sized panels @4k, while it'll take a 1080ti/sli setup to run it in many games, I'm hoping it'll be more playable in AH3 while in 4k for me. That's when I'm not using the Rift at least, which I find fun, but I'm going back to LCD frequently I find lately.