I have the h-bomb (rip curls heated vest) and is the best winter surfing investment I've ever made. I wear a 4/3 all season and only use a 5/4 when doing a double session or surfing back to back days. I've basically had the same experience as sandyhook. If you have the funds I recommend getting one.
Not trying to sound like a total bad-arse, though I definitely am one, but I'm not really seeing why a heated vest is necessary. If you're trying to stretch a 4/3 into winter suit then maybe, but man...a good 5/4 gets you in the water comfortably when the air is 10F and the water is 35F. Seems like more of a luxury thing, which is fine.