I'm surprised it took us this long to compare to the decathlon, it seems like an obvious thing to do.
Top decathletes (Eaton, Hardee, etc.) are of similar athletic ability compared to the NFL players we're talking about, and also train somewhat similarly - a lot of weights, a big focus on explosiveness, little focus on endurance (though likely MORE than NFL players), but all in all more well rounded, AKA better at the mile.
Eaton is 6'1, 185. He would be around 4:25 - 4:30 fresh (4:14 1500 PR). He's arguably the most athletic human-being in the world right now.
Hardee is 6'5, 210, similar to Kapernick and Alshon. He would be right around 5:00 fresh (4:42 1500 PR).
Sub 4:30 seems unlikely for any NFL player.