Enterprise was pretty conscious of canon literally from the first season. The major difference is that the fourth season increased the amount of references to canon, but also built on the previous season's handling of canon - the reintroduction of Andorians and Tellarites, for example, after not featuring in any significant way in the franchise in decades.
Hell, I'll still remember falling out of my chair when I realized the series made a reference to the Axanar, and that was in the first couple of episodes. It was pretty obvious that Berman and Braga were quite conscious of the canonicity criticism from the beginning and made an effort to genuflect towards that (just as they pared down the technobabble from Star Trek: Voyager levels for similar reasons).
Even in the more notorious episodes to skirt around canon, like the Ferengi one, made a point of acknowledging tradition - when it first aired, I remember angrily complaining online during the ad breaks (good days) not that the Ferengi were there, but their admittedly ridiculous laser-whip weapons from "The Last Outpost" were nowhere to be seen.
Only to eat my words when the episode did briefly use the whip later in the hour.
This isn't a defense of the B&B years, which for me were so terrible I basically stopped watching the series, but it wasn't canon violations that drove me away - it was the terrible writing. It was bad as a series, not out of its continuity issues. More important then the epic amounts of fanwank Coto gave us, his year also had a palpable sense of fun.
The big difference is more, because it was more reference-conscious, people liked the last season more than the first two (and this includes myself), not to mention the fresh blood that Manny Coto provided.
