I sit about 31 inches from the monitors and they were both running at 2560x1440 (native for the 1440 monitor and scaled to 2560x1440 for the 4K monitor).
That makes some sense, at least to a certain extent. I sit at 22" or so from the monitors. This is a key factor:
27" 2560x1440 monitor - 109 ppi - Retina at 31.6".
32" 2560x1440 monitor - 92 ppi - Retina 37.5".
27" 3840x2160 monitor - 164 ppi - Retina at 21.1".
32" 3840x2160 monitor - 138 ppi - Retina at 25.0".
Note though that some people don't mind low ppi monitors if they are running at the native resolution. For example, I liked my 30" Apple Cinema HD Display:
30" 2560x1600 monitor - 101 ppi - Retina at 34.2".
The 30" ACD was pixelated, but the text was very crisp nonetheless, and at 101 ppi I really liked the default text sizing, more so than Apple's current default for the Apple Studio Display which is 109/218 ppi. The text on these is slightly smaller, enough that I don't like it as much.
My holy grail would be a 30" 5120x3200 monitor, which would be 201 ppi and Retina at 17.1".
I currently run a 28.2" 3840x2560 monitor, which is 164 ppi, Retina at 21.1", perfect for my 22" seating distance. Truth be told, even at this supposedly Retina 22" seating distance I can tell it's not quite as crisp as my 218 ppi 5K iMac, but it's close enough that I don't mind.