Yeah, at this point there's no way SHIELD gets cancelled before completing four seasons. That's how many episodes you traditionally want for syndication/streaming packages, and ABC will do whatever it takes to drag the show to that number.
But I have a very, very hard time imagining SHIELD being renewed for a fifth season after next year. In fact, I could see them putting the show on Fridays next season and just burning it off.
And yeah, Most Wanted and Agent Carter could both be in serious trouble. Carter will only have 18 episodes after this season, which isn't even a full regular season. Syndication won't save it, so the ratings will have to stand on their own this year.
I was utterly baffled when ABC ordered the Most Wanted plilot. Agents of SHIELD has bled viewers throughout its entire existence and at this point is getting beaten handily by Flash, Gotham, and Supergirl in the weekly ratings. And Agent Carter put up okayish numbers last year. It was a very borderline show that could have gone either way (I actually thought Carter would be cancelled last year; it was the only prediction I made about a comic book show that I was wrong about; I predicted Constantine and Carter would be cancelled and Gotham, SHIELD, Flash, Arrow, iZombie, Daredevil, and Powers would get renewed).