I can't speak for experience with a bear tank and four melees, but I do have some experience with a bear tank and am getting more every day.

With four rets, your dps will be right on top of the bear. This is good, because the DPS will always be within swipe range. So even if one of your rets gets aggro, you don't need to run back to some line of DPS to pick up the add, or growl, or whatnot. You can just keep up the swipes.

As a bear and four shamans, that's my biggest weakness as a tank. Adds that I don't swipe in time and start picking on my healer, or something.

I say give it a shot. It's not something we've seen around here, and it's always fun to try and out tanks. I like bears.