Update: Thanks everyone for the replies. I couldn't decide what to do so I ordered multiple sizes. They arrived today so I thought I'd share what I found.
Pants: The size 4 pants are considerably wider than the the 4T both in the waist and in the leg. DS is wearing a 3T now (I was buying ahead for next winter since the fleeces were all $4.XX over the weekend) and I can't imagine that his skinny waist will be able to hold up the size 4 next year. The size 4 pants were also a good bit longer than the size 4T. BUT, the length from the waist to the crotch was the same in both the 4T and 4. I guess this is because they're assuming that a 4T child is wearing a diaper and the size 4 child is not. I did think it was interesting that the size 4 was so much wider though -- it's like they're expecting the diaper to take up length in the 4T but not width.
Now, the size 4 shirt was also super wide. DS is now wearing a 4T in TCP tops so I wasn't sure if he'd need an extra small (size 4) or small (5/6) next year. Well, now that they've arrived, I don't think there's anyway he'll take the small next year. The extra small is about an inch longer than his current 4T so not much bigger there but it has much longer arms and is WAY bigger in the body. I think the 4 is plenty big and that he'd swim in a 5/6 next year. My little guy is really going to need to fatten up to fit into these clothes as it is! He's tall so I think he'll need the length but wow, I just can't believe how much wider TCP's clothes get when you move out of toddler sizes!
Hmmm, so here's a question. I'm going to return the size 4 pants and keep the 4Ts because I think the 4s will be too wide next year and may not be long enough for the following year. But what would you do about the tops? I'll keep the extra small (size 4) but should I keep the small (5/6) sizes too? I don't think they'll fit DS next winter (most likely the next) and if I hold on to them DS will have some of the same tops in two different sizes. So when he grows out of a top, his "new" top would just be the same top in a larger size. Would you keep the size small tops knowing that or would you be sick of seeing your kid wear basically the same thing two winters in a row? I'm leaning toward the latter but curious to hear what you think...
Is the 4 any bigger than the 4T or are they essentially the same size?