The TC4 is a twin brother of the SD from Yokomo. Which is the same platform used for some of the drift cars on the market today from Yok (and others).
Some time ago, Yokomo released a very nice gear diff for these cars which fits perfectly in the old SD. I haven't tried it in a TC4 and from pictures it looks the gearboxes are somewhat different, but perhaps worth a shot.
I have also successfully modified a Tamiya TB(?) geardiff to fit the SD, but this is more fiddly and involved and with the Yokomo diff on the market there's no point for me to pursue this avenue, but for the TC4 it may just give the needed size to fit in the g'box case.
There's also a Spec R diff for the TB platform, which has a tapered casing and may fit easier than the above. Again, I have never tried this.
Team Greasy Weasel
The best upgrade to any car is some driver skill.