The broken mount in the car.
For step-by-step instructions on how to remove and replace the old mount (and lots of other great GT4 information), see Kris Carter's webpage here: http://www.gtfours.co.uk/
The broken mount out of the car. Cut the remaining rubber out of the mount with a hacksaw. Scrape, dig, and gouge any remaining loose rubber with a screwdriver or other suitable tool. Clean with a wire wheel. If there is any grease or oil residue on the mount, clean it with brake cleaner (my all purpose surface prep cleaner).
Here the mount is cleaned and ready to be taped up.
I used aluminum tape, available in the heating section of Home Depot and other home improvement stores.
Notice that I have pressed the tape tightly onto the mount to prevent any leaks.
I used urethane casting compound from McMaster-Carr. The $23.64USD price for one pound of urethane looked like a better deal than the $154.93USD list price for a new diff cushion from Toyota. You can save about $30USD if you order the cushion through http://www.toyotapart.com/ (part# 52291-20010).
I chose the Flexible Urethane, Shore 80A, part# 8644K11, for it's minimal shrinkage properties. Excessive shrinkage during the cure process may cause the urethane to pull away from the mount. For best results, follow the mixing instructions exactly.
Place the taped-up mount inside a suitable container to catch any leakage. The viscosity of the urethane mixture is a little thinner than fiberglass resin, so it will seep through small gaps if there are any. I was fortunate to have no leakage from the tape. Also note that there is enough urethane to fill more than one mount. If your motor mounts are easily accessable, you may consider doing them too. I just poured the excess urethane into a plastic bottle and I plan to keep it as urethane bar-stock for whatever applications I can dream up in the future.
When it is all done, it looks like this.
Note that the urethane needs 7 days to fully cure, so if your GT4 is your daily driver, you will need to schedule a week of down time, or source a spare mount to use in the mean time. Also note that you will need a well-ventilated place to work with the urethane, and the ideal temperature is 77*F.
Disclaimer: Read and understand all of the health and safety warnings before opening any of the urethane containers. Liquid urethane is some nasty stuff. Use the information on this page at your own risk. If you fall over and die, don't blame me.
Update - 08/16/04:
After about two years and ~17,000 miles the mount looks just like in the photo above (except for the expected road grime). If I use a pry-bar to lift the differential, I can see that the urethane has loosened from the outter frame of the mount a little, but it isn't noticable while driving.