It seems that this question can not be answered. Acording to the Heinlein Concordance

John Martin
One of the Four Heroes of the Interplanetary Patrol, whose name is always called (the answer given by a designated patrol member) at every muster. (The reason he is included was not given.)

and

Wheeler (no first name)
Interplanetary Patrol hero, one of the Four Heroes whose names were called in every muster. The reason for his legendary status was not given in this or any other story.

It seems that this question can not be answered. According to the Heinlein Concordance

John Martin
One of the Four Heroes of the Interplanetary Patrol, whose name is always called (the answer given by a designated patrol member) at every muster. (The reason he is included was not given.)

and

Wheeler (no first name)
Interplanetary Patrol hero, one of the Four Heroes whose names were called in every muster. The reason for his legendary status was not given in this or any other story.