I have a good sense of Archer, Trip, and T'Pol right from the first season.
Archer: Well-meaning pioneer who gets in over his head. He has something to prove to the Vulcans because they screwed his father over and he wants to show what Humans can do for the Interstellar Community.
Trip: "Aw shucks!" southern gentleman who loves his engines and a piece of steak. Best buds with Archer. He wears his being a Human on his sleeve. He's also the comic relief. Sometimes he says things that Archer can't.
T'Pol: A Vulcan who starts off very stuck up and doesn't want to be around a bunch of knuckle-dragging caveman Earth people. Quick to point out how Archer the rest of the crew are messing up, quick to mention how Vulcans do things, but then she eventually comes around to Archer's way a little bit and starts to warm up to the crew, even though she still stays generally aloof.
Reed: He's the James Bond character trapped in a Star Trek series. He's all about gadgetry and weapons. Keeps to himself mostly, and is all about naval tradition. Stays this way for the next three seasons. Would've stayed this way had the series continued another three.
Phlox: Nice guy, cultured, has a lot of life experience, likes to explore, thinks outside the box when it comes to medicine, and that's largely the way he stays.
Mayweather: He grew up in Space, loves to pilot ships, and... and yeah! This is how it is all four seasons. Wouldn't have changed it if went seven.
Sato: She's nervous, is more competent than she realizes, and great at translating languages.
Had ENT been given Seasons 5-7, it would've done more with Archer, Trip, and T'Pol, but the other characters would've stayed the same. Or mostly the same. They would've shown more of Earth's alliance with Vulcan, Andor, and Taler. They probably would've shown the Earth/Romulan War. There would've been more TOS (and some TNG) stuff sprinkled in.
But as far as getting to know the characters? They were pretty much as developed as they were going to be. Three more seasons wouldn't have changed that.