There are several guys with the 4.8is running 18" winter wheels, so I'm not terribly concerned about that.

For the poster who states that the rear brakes are the same, they are not.

The calipers, rotors, and pads are different. I've put together this table...

You need the calipers, caliper carrier, rotors, pads. The master cylinders are the same.

Sorry about the horrible legibility of this...formatting doesn't do well here.

REAR

4.6is 4.4i

Disc 34216756849 34216765458
caliper left 34216757055 34216768443
caliper right 34216757056 34216768444
bracket 34216757058 34216768438


FRONT

Disc 34116756847 34116765457
caliper 34116757053 34116773132
caliper 34116757054 34116773131
bracket 34116757057 34116768437
Pads


Master cylinder 34316757743 34316757743



Please note that the caliper numbers are the shell - they do not include pistons or seals. My local parts store cannot get the calipers.

Based on the prices I see, this is probably cost-prohibative to a lot of folks. You basically need to find a wrecked 4.6is or 4.8is and be able to steal everything that you need off of one, in order to do this economically. That being said, it would probably be worth my while (and pay for itself in the long run) to upgrade just the front brakes. After all, they do most of the work, and since the master cylinder is the same, I don't think you'd significantly un-balance the braking system.