Not my MK4, but fixed stripped threads in my housemate's son's MK4 Jetta 2.0.
As a novice driver learning how to take off on an incline, he managed to lurch and jerk hard enough to strip the threads out of the motor mount bracket that bolts through the timing cover.
Instead of replacing the whole bracket (and having to remove the bracket!), we ordered a Timesert repair kit and 19mm long inserts.
The shank of the Timesert drill bit and shoulder reamer turned out to be barely larger than the large tap holder I own, so we had to get creative.
Using a ratchet, extensions, and a T55 socket inserted into a 3/8" drive 10mm socket, we were able to drill the damaged threads out, ream the shoulder, and then tap the new threads in the holes, all with the mount bracket still in place on the engine. Using a Big Gulp straw duct taped to a shop vac hose allowed us to clean out the holes regularly. (The straw also gave us a musical interlude, every time we turned off the shop vac!)
It's now reassembled, and the repair kit cost less than a new mount, as well as being on hand for any other 12mm thread repairs that may come up. (My TDI's motor mount being a prime consideration. It might get them "repaired" just as a preventative, when I have the mount off for the timing belt, soon...)