I assume you are referring to Shakespeare's As You Like It monologue in Act II Scene 7, lines 139-166.
The fourth stage is "the soldier":
Then a soldier,
Full of strange oaths, and bearded like the pard,
Jealous in honour, sudden and quick in quarrel,
Seeking the bubble reputation
Even in the cannon’s mouth.
It's helpful to remember that the stages are (1) infant, (2) schoolboy, (3) lover, (4) soldier, (5) justice, (6) unnamed, but let's call him the "retiree," and (7) also unnamed, but..."old man."
The soldier is the phase of life where men are strong and often eager (and able) to prove their manhood. They develop the ability to grow a beard (and to this day, will often wear it long as proof of manliness), and--not to paint men with too broad of a brush--will happily fight to defend their (or someone else's) honor.
The man in Shakespeare's fourth stage of life isn't necessarily a soldier. This is a conceit to demonstrate how volatile men at this stage of life can be. He is "full of strange oaths," meaning that in his drive to defend his manhood, he is willing to swear to do lots of things, however odd to common ears. He is jealous of his honor (which I think is self-explanatory), and willing to fight (as the cliche goes) at the drop of a hat.
He seeks "the bubble reputation," meaning he wants a great reputation quickly, without really having to work for it. (The trouble with bubbles, though, is that they are easily burst.) He will fight "even in the cannon's mouth," meaning that he takes foolish risks, all in the name of honor.