I just went from a three monitor setup (L to R) 19" for email, 27" 2560x1440 main (photo editing) and 24" 1080p for web / watching video to the 27" on the left and a new 32" 4K on the right.
My eyes are getting old, and I sometimes use reading glasses, but I can read (unaided) everything in 4K at 100% scaling. For some things, 125% is more comfortable though.
I'm using PS and LR at normal scaling. IQ is definitely crisper on the 4K monitor than any other monitor I've had. Contrast is better too. Overall, these two took less desk space than the three.
I do miss the three, but I am working on getting used to having email and the web in separate side by side windows on the 27".