Dear Big Slick,
You’re starting the AT in April, have a rain jacket and are looking for another, additional jacket, this one for warmth.
My advice would be a light down or synthetic insulation jacket, perhaps synthetic in view of the amount of moisture one runs into on the AT.
That being said, I’d recommend the Mountain Hardwear Chugach, with Polarguard 3D and a ton of useful features, although ultimately light weight (20 oz?–check their website), as your bottom line. If you want to go warmer, do so, but don’t get anything less warm for the Spring. A light rain, or wet snow, in this thing, around camp, due to the sythetic insulation, will not pose any kind of threat to you.
Given your relatively light other layers, this is what I would do if I were you.
Good luck.
Sincerely–Conan.
Conan