My uninformed guess is that because so many people are accessing the 4G network for the first time (because of the Bionic's release), it's messing with the 4G network system-wide. I'd give it a day or two to see if it settles down.
Are you driving when it does it?
I think it's supposed to do that, right? As the 4G signal comes in and out, it switches back and forth? I don't know for sure as I am not in a 4G area yet...just postulating!
Having a pretty rough thunderstorm in the area right now and signal drops in and out right now for me. Anywhere from full 3G bar to 2 bar 4G when it stops raining if it still does this might have to return this thing...
I wouldn't return it right away. Mine was doing that at first (clear weather), but it's settled down and has held a constant 4G signal. If it keeps doing that, though, you still have 2 weeks to return it