Carl,
Your use of purlins on 4' centers implies that you're using principal rafters. And principal rafters and step lap rafter seats don't often work out well, because you have to take too much meat out of the plate.
Like Tim said I would stick with continuous top plates, common rafters, and nail 2x4 purlins over the rafters, metal roofing over that. I like the purlins at 20" OC because it's a little easier to crawl around the roof.
If you want to use clapboard siding, you may want to consider a Dutch frame, with smaller bents spaced closer together. That way, you don't need any studs, just claps right on the posts. Or use girts if you want vertical siding.
I have plans for a little 3 bent 16x20 if you're interested. I could send you a picture as well.
Brad
