Almost forgot about this one.
Well, if you look at this graph one way it looks like 4:00 is kind of an asymptote.
But then, 4:10 was looking like an asymptote from 1989 to 2003, and then it wasn't. Same kinda goes for 4:20 at one point.
So I will say yes, there will be improvements in technique (the freestyle stroke has evolved since I swam) and technology (sport science has made leaps and bounds since I swam) such that he (or someone) does break 4:00. There could even be rule changes (see backstroke flip turns) that cause a further drop.