Be careful though if you are using rewire. Reason 4 installs a new rewire engine
that is incompatible with Reason 3. I tried this:
Had reason 3 installed. Rewire worked fine with Live.
Downloaded and installed reason 4 demo to give it a spin.
Reason 4 rewired fine to live.
If I then opened up a project using Reason 3, I could no longer
rewire to Live. Deleting the rewire folders and re-starting Reason 3
solved the problem.
BTW, this was done on a Mac, things might be different on a Windows
box.