New monitor

27 full HD should look fine. Text will be larger, everything will appear magnified compared to HD on smaller monitor, but at 27 inches it should not appear overly magnified to the point of pixelated images. If you go a lot larger at HD images may appear pixelated at a close viewing distance. If your smaller monitors are less full HD, then going to full HD will get you more real estate on screen (e.g., more faders across screen) but text may appear smaller depending on relationship between increase in screen size and increase in resolution. A 4k monitor  will get you  roughly 4 times as much real estate on screen. But if screen size is smallish, the text, etc., will appear very small. The screen size diagonal of a 4k monitor has to be double that of a HD monitor to get 4x the real estate at the same text size. IOW, if your 20 inch monitor is full HD, a 40 inch 4k would get you the equivalent of having 4  x 20 inch monitors all on one (rather large) screen.