Ok, so now looking side-by-side the text is definitely more crisp on the MacBook Pro. Having said that, the text on my Dell does not bother me, especially sitting at normal distances, and I do not notice it in normal use.
For the OP, the hard part about 4K is that it does not scale well at 27". At native resolution, everything would be pretty small. Using the ideal Retina resolution (1920x1080 for 4K) makes everything too large. The resolution for a 27" should be 5K where the half resolution is 2560x1440 which most consider ideal for the 27" size. You can do non-integer scaling to make 4K appear like QHD (1440) but that can come with a performance hit and produce results that are not as good. If I were to go 4K, I would look at either a 24" monitor where I could do half resolution or a 32" that I could run at native. Otherwise I would look at 5K for 27" or settle for QHD (1440) for 27" which is ultimately what I did since there are not many 5K displays and I find 27" to be the ideal size.