First off, I'd consider the Halo 3 announcement trailer to be a full-fledged trailer, or at least a teaser on steroids.
Secondly, I disagree that we've gotten more on Halo 4 in this point in its lifetime than Halo 3 at the same time. If you're talking about quantity, sure, we've gotten more on Halo 4 than Halo 3, but when it comes to overall substance and content delivered? I don't think so.
Let's take a look at the two trailers, starting with the first. It's a teaser. We got some hints and teases when it comes to the weapon the MC uses and his thruster thing. But that was it. Nothing new was revealed about the plot and we weren't given any idea of what the final game will look like.
The concept art trailer is definitely better on providing the fans the direction 343 wants to take the story, but we weren't given anything concrete, at least nothing as much or more concrete than anything revealed in the Halo 3 announcer, for us to really latch on to, aside from the not-very-helpful image of whatever at the end. We have an idea of what 343 is aiming for in the look of the game, but because it was only concept art while cool ultimately doesn't do a better job than the Halo 3 announcement trailer of giving us a good idea.
Now, the Halo 3 announcement trailer provided us with all of that and more. It gave us a tease in the redesigned assault rifle. It gave us real story information to latch on to in the form of Cortana's cryptic, Gravemind-induced messages, and the mysterious, huge Forerunner structure in the middle of Africa. (With subtle details like the movement of the Covenant ships: at one point the Covenant ships were moving toward it, then when it comes to life, they move away). And, probably most importantly, we were given a good, though ultimately deceptive, idea of what Bungie was aiming for with the look of the game. When you compare the trailer to the final game, it's mostly the same aside from the lack of AA, MC's armor texture, and the fins of the Forerunner portal are a bit murky.