[A930Rocket]

If the rafters are 2x4's, then they are really over the limit by todays standards, especially if they are 24" OC. The 1x4 decking and roofing are weighing it down. Asphalt shigles might make it worse due to weight, without support.

Adding plywood, osb, etc. to the roof won't do much without the rafters being supported or replaced.

You might be able to a few things, but I'll leave it to an enigineer:

1- Replace the rafters and/or 1x4's decking and brace to proper load points below.
2- Scab on 2x6's or 2x8 to the 2x4's from the ridge to the outside plates. You'll need to replace the ridge with a 2x8 or 2x10, depending on which rafter you go with, or maybe scab on a 2x to the bottom of the existing ridge. Current code requies it do be as deep as the rafter.
3- Add more bracing (a lot more) and collar ties.

One problem without replacing the whole roof, is getting the roof flat by bracing. It will take a lot of work to get it flat.

I'm sure there are more ideas out there and others will chime in.