Hi John, Mark is dead on about the thermals, as he is with all his advice. Definitely check out his webpage; he has a ton of great info, and has more product reviews than any personal page or blog I've seen. Thanks Mark for this great resource!
Anyway, your thermals, or any other clothing you carry, should have ZERO cotton. The old adage "cotton kills" is true. Synthetics or wool are the way to go; synthetics are cheaper and dry faster, which is a huge advantage on the NC AT in spring. It will be wet, and you'll be slightly damp all the time. With cotton, it would be saturated within a few hours, and you would be hypothermic a few hours after that.
As I mentioned, it will be wet, April is typically a pretty wet month in the mountains around here. You will need raingear that is 100% waterproof. If your windbreaker isn't 100% waterproof as I suspect, that's OK. Get a silnylon poncho to accompany it. The windbreaker will work fine for what your typically dealing with: wind, damp fog, light drizzle or snow flurries, and wet brush. When the real rain starts, just break out the poncho and wear it over your windbreaker. The poncho will come in handy around camp as well; if you are using a tent, you can pitch the poncho as a kitchen canopy. It will also keep your pack dry during downpours.
Also, in early April you are going to need an insulation layer. A fleece jacket or pullover would work fine, a synthetic puffy jacket even better. Also, you'll need a warm hat and I would bring some liner-type gloves; Smartwool liners and various makers' Powerstretch gloves have always worked well and would be a good choice.
Don't forget your feet! Socks should have no cotton content either. Again, wool or synthetics. Wool works better for me personally. On a longer trip in wet, potentially cold environments, I carry three identical pair. One on my feet, one either drying in my shell (when it's raining) or on my pack (when it's sunny), and the third in the sack with my sleeping bag. These will be bone-dry and I don't put them on until I'm camped and in my bag. Worth the extra 1.5-2.0 oz. to have warm, dry feet at night.
All your clothes are gonna get pretty smelly after a week or so unless you're wearing all wool. Carry a packtowel, a little Dr. Bronner's, and maybe some Gold Bond lotion or powder. These will help mitigate the odor for a little while, and also help keep any kind of fungal infections or rashes at bay.