It sounds like a great series! I’m not exactly sure what you’re looking for, but I’ll give it a shot.
You said Hero3 is in books 1 & 2, so his character has been established – is it firm or has he just been mentioned? Who is the heroine in book 3 and what’s her GMC? What kind of missions do they go on that he thinks he’ll find love? If the only reason he joined is to find love, I think they really, really need to hate each other in the beginning!
Hero4: Is it possible he joined the military to get away from poverty, can’t stand the life, and wants out as soon as possible (hasn’t found the right assignment maybe) so he’s obsessed with saving for once he gets out and he joins the program. But in joining, he finds the “right” place for him. Who is his heroine?