"U" in this context means "undesireable," which is how 4A and 4J are coded. The reason? They are right next to the J class galley and, as such, are more susceptible to noise and light than other seats. The AAgent misunderstood the reason the seats are coded "U," but I suspect this is because they look, on a seatmap, to be right up against the back wall. This is not the case and they do recline fully.

Personally, I would take 4A/J over any center seat, regardless of FEBO.