Lebron is incredibly athletic, but doesn't possess the body type or specific training for 400m running.
No one runs world class times without a world class physique and world class training. It doesn't matter if L. James is a better all around athlete than K. James, he isn't going to run 45 seconds, period. The same thing goes for 46-48ish. Those are times that people only really reach with specific training (and I'm not talking about a relay 48.high split which is actually worth like 50.5). No matter how athletic and explosive he is, he's not hauling that physique around a track with no specific training or loss of unnecessary (for 400m) muscle bulk.
On the other hand, people who think he would run 55-60 are also clueless. The man is a freak of nature athletically. Plenty of athletic high school boys step on the track and bang out a 400 in 54-55 with little to no specific training. This website is predominantly long distance runners who likely have little explosive or fast twitch ability. Thus, a 400 of 50-51 seems very fast, and it is, but only from the viewpoint of an endurance athlete. For a 400 specialist, it is abysmally slow. I wouldn't be surprised at any result from 49-53, and would probably put money on 51.5 if I had to.