A piece of porcelain broke off my bridge (the four front teeth) can the piece of porcelain be fused back on to the bridge?

You have 3 options depending on where the piece is broken off:The best option is of course get a new bridge, the second best option is see a cosmetic dentist to repair the fracture via dental bonding, and check your bite. I have treated many patients with dental bonding for repair of porcelain fracture, and they are still doing well five years passed. There is possibility of minor staining, since bonding is not the same material as porcelain, however polishing the bonding , will make it look like new again.If the broken piece is on the edge of tooth where you tear food, then your other option is possibly to have a partial veneer, substitute on the broken edge which will cost you again way less than changing the 4 unit bridge.

If the piece of porcelain fits back on well it can be bonded back into place.  The other option is to use normal dental bonding material to repair the break.  Longevity of the repair can be an issue depending on amount of force being placed upon it during chewing or sleep. 

Despite our best efforts, this will be a temporary fix.  It may last a few days or several weeks, but it is not the same as heat fused porcelain.  Bonding the porcelain, even if it "sticks" will stain and be an esthetic failure at the very least.  Most of the time the repair simply won't stay, but should buy enough time time to get the bridge replaced.