It isn't quite clear what the situation is by your description. If you are saying that you have vertical framing in place that goes between the top plate and the rafters that was put in on the flat then you need to just nail 2x4's to them perpendicular that would bring the wall thickness to 1.5" plus 3.5" = 5". You could shim between them with 1/4" or 3/8" plywood to bring the wall to 5.25 or more.
If this doesn't answer your question then you should probably try to get a little more detailed with your description because we can't see it from here.
If you have cathedral ceilings you might want to maximize your insulation by "building down" the ceiling for extra insulation. See: http://www.jlconline.com/jlc/archive/energy/cath_ceiling/index.html for an excellent article. You will also need to budget money for decent ceiling fan(s) and have the wiring and boxes installed before drywall. It is almost impossible to be comfortable in a tall room without the proper fan(s). I have a 19X20 room with a 12' ceiling and I have a 52", 14 or 15 degree pitch, 5 blade Casablanca and it is perfect. They have a neat "intellitouch" control system that only needs two wires. Cheap fans don't work, good ones cost a lot.