I've been searching for a swap like this on google for the past half'n hour, and I have come across zero. If you do this, you may be the 1st one. I can't imagine it being as bad as the way Rob here put it, but it may be. I don't see the weight difference being anything more than you can handle, but what I do forsee is the height. I'm pretty sure the TR4 engine is taller, and thus would not fit under the stock bonnet... I don't know how much taller for sure, but it may not even fit under the GT6's as well.
If you have the TR4 engine and a spitty to play with, then go for the gold. And as long as you ask nicely and post pictures then I'm sure everyone of us will be interested and help with input. Besides, all the TR's (2-8) have been faster (in a straight line) than any of the Spitfires ("The Bomb"-1500) so if it comes out well, it could be a blast. Any way ya look at it, the swap should be a fun project!
If ya don't already have this TR4 engine, and your looking for an awesome swap, then I agree with Rob, the Mazda 13A or 13B rotary is the most awesome way to go. Sure, it's still high maintenance but what is out there that is pretty and isn't?