don't get me wrong...if everything comes together & the conditions are perfect (ie: little or no wind, sunny, warm air temp, minimal duckdiving), then a 4/3 is feasible in january or february. but how often does that happen? the best winter surf comes when it's windy & cold, by & large, & (at least here in nj) tends to involve a lot of duckdiving...something that gets pretty "exciting" in a sub-40° water in a 4/3. it's even more "exciting" in sub-35° water.
the guy is located in rhode island...those of you suggesting that a 4/3 is practical all winter up there are potentially endangering his life w/ crappy information.
It really depends on the deep ocean currents. New England gets a pretty good shot of the Labrador coming from the Arctic, but the severity depends on the greater climate like El-Nino/La-Nina. Notice how far up and close to the coast the Gulf Stream remained this summer, bringing warmer than usual water further North and mixing more North of Cape Cod. I think this could keep the water warmer for a longer time up there as long as the eddy keeps occurring in the same general area. However, RI can get some serious upwelling too--or at least that's what a buddy in Narragansett has told me.
Short answer--as long as you feel comfortable but I wouldn't push it too far. Not too many breaks in Rhodie you can get in and out of very easily and nearly impossible if someone has to drag you out.