I'm not 100% certain, but believe a football injury caused him to be classified 4F.
You all might enjoy this John Wayne annecdote from the Internet Movie Database:
One evening before a shoot he was trying to get some sleep in a Las Vegas hotel. The suite directly below his was that of Frank Sinatra (never a good friend of Wayne), who was having a party. The noise kept Wayne awake, and each time he made a complaining phone call it quieted temporarily, but each time eventually grew louder. At last, Duke appeared at Sinatra's door and told Frank to stop the noise. A Sinatra bodyguard of Wayne's size approached saying, "Nobody talks to Mr. Sinatra that way." Wayne looked at the man, turned as though to leave, then backhanded the bodyguard, who fell to the floor, where Wayne knocked him out by crashing a chair on top of him. The party noise stopped.
LOL! I can almost see it :)
I didn't know Wayne and Sinatra didn't like each other. Hm.
I loved the Wayne story....he may have been 4F but one of the reasons was he had several children at the time and that too kept him home, although, he could have volunteered..