First thing you notice is much warmer whites on the Nexus 5. In the original link below, you can also see the subtle difference a higher PPI makes on text. Even if you don't see pixels in either phones, the text is cleaner on the Nexus 5.
Original:
http://i.imgur.com/Z6lMhoO.jpg
One interesting thing is that the Nexus 4's screen shows more stuff horizontally. Usually letters, one or two more words. I presume it's due to the ratio of resolution. (Nexus 4 is 1280:768, instead of 1280:720) You can see this from AnandTech forum app above, "SSF, Notebooks, Pre-built/Barebone PCs" sub-section label. In other words, the Nexus 4's screen is wider when viewed vertically, and can display more text in a given line than the Nexus 5. This is consistent system-wide. Not sure how much of a big deal this is on ~5" screens, though.
So which screen is better? Nexus 5 by a lot. Only thing the Nexus 4 beats the N5 is contrast, especially darker black.