The important parameter for comparing is pixel pitch, and how much that changes between what you are comparing.
A 4K TV/monitor is equivalent to a 2x2 array of Full HD (1080p) screens, making the 4K TV that would have the same size pixels as your 27" = 2x27" = 54", so the 50" would be much closer to the same pixel pitch as your 27" than a 40".
You obviously chose a FHD 27" for a reason, rather than a 2560x1440 27". If you preferred the larger pixels, then 50" is your obvious choice. A 40" 4K TV would have almost exactly (within 1%) the same pixel pitch as a 2560x1440 27", so anyone used to using an iMac would have no trouble with a 40" 4K.
Note that TVs are not necessarily of the same grade of screens as premium monitors, which, usually being IPS, have viewing angles of 178 degrees. TVs typically have lower figures than this, so when watching a large flat screen up close, the viewing angle for content at the sides of the screen can be quite large, so that colour shift can start to become noticeable. This is why I bought a curved screen 55", as well as it not requiring noticeable focal changes between content in different parts of the screen. On flat 48" screens (and less), the effect is less noticeable.
However, just because a TV might have an IPS screen, that does not mean it is good. We were originally looking at LG IPS TVs, but when I read reviews of them on sites that did proper in-depth TV testing, they commonly cited poor colour compared to the Samsungs, and that it could not be made right. On the Samsungs, I had to make adjustments to get better balanced colour, but at least they have comprehensive controls for that, though I didn’t have to go to those extreme ones on either of ours.
I hope this helps.