Really, "it's up to your eyes".
Only YOU can decide whether 4k is "worth the money" (and that will probably be AFTER you've gone out and bought one).
I'm almost 70, and my eyes aren't what they used to be.
I don't really perceive all that much difference between my old 2010 MBP (non-retina display) and my 2015 MBP (retina display).
Yes, the new one "looks a little better", but "the difference" is not really all that much to me any longer, due to aging.
I currently use a 27" 1080p display, and it "looks good enough to me".
Others (mostly younger) will howl that that a display of that size and resolution is "too grainy". Not me.
If you're going for a 27" display, the difference in pricing between 4k and 1080p displays isn't all that great any more.
You might just spend a little more, get 4k, and be done with it.
My next display will probably be 32", 1440p.
The pixel size will be smaller than what I use now, but hopefully still usable and readable to me...