1. 4k =/= 2560x1440, 4k = 3840 x 2160; in case you have it confused
2. My last monitor was ViewSonic VX2770-SMH, my personal opinion is that, while 1080p at 27" doens't look bad, it was on the threshold of me being able to notice the individual pixels (hence 28" 1080p was a no-go for me), hence I opted to either go for a 24" 1080p or 27" 1440p. I went for the latter, and I like it for the PPI, lack of pixel lines and I could put more things on my desktop.
My personal opinion though. Some find even 23" 1080p to be pixelated, I am used to 24" 1080p.