You could put a 5 weight line on a 4 weight reel, you just wouldn't be able to use as much backing.
I suppose click-pawl may be inherently a lighter design than a disc-drag, but there are plenty of reels out there with a disc-drag under 5 ounces. Personally I think a disc drag is better. (Even though it isn't needed much with the fishing I do, and the fishing it sounds like you are planning to do) If I had two reels of exactly the same specifications and one was disc-drag and the other click-pawl I would take the disc-drag any day. I fish a click-pawl though, just because I never ever use the drag so what do I care. The large arbor thing will kinda limit your choices a bit, but if you must have a reel under 5oz I think if you open yourself up to disc-drags you may find a few more that are out there...I spent $30 on my reel (After owning much more $$ ones) and have never looked back....It works just as good at storing line as a $400 reel :) It is about 2oz heavier though, and not a large arbor.