Hi Jeff: You say "framed with full dimension 2X4's"...do you mean the rafters are 2X4's? If so, how sufficient that is or isn't depends on a couple of things. How far are they spanning and what is the pitch? Are they supported at any point along their length?
I would say that on a 2-4 pitch roof that spans only 12 feet or so, the old full 2X4's, which are probably straight grained pine, are probably strong enough. But the best way to tell is to put a 6 or 8 foot level on the bottom of the 2X4 and see how much sag they have. If its less than 1" in a 10 foot run, you are probably ok. More than that and I would consider putting tension into the existing 2X4 rafters (using a small bottle jack) and sistering on straight grain, kiln dried 2X4's on each side of the existing old 2X4's, securing them with a good construction adhesive and lots of 3" screws.