The first thing to do is to determine whether the water is in the aluminum section or the comptip section of the mast. The two sections are isolated by various foam plugs. You can do this by placing your ear right against the mast and having someone tilt the end up and down. You will hear even very small amounts of water (and definitely 4 pounds of water) sloshing around inside.
Pulling out the rotator bolt/rivets will probably not allow the water to drain from the aluminum section. There is a foam plug in the mast that the rotator bolt passes through and seals the bottom of the mast and the bolt hole. If your mast has a sail feeder at the luff track opening, you should be able to remove those screws to drain the aluminum section. Otherwise, you can drill a small hole in the mast around that area, drain the mast, and then seal the hole with a pop rivet or self tapping screw and silicone.
If your comptip is leaking, then you need to pull off the head cap to drain the mast. This requires drilling out the six halyard hook rivets to remove the head cap. I had water in the comptip of my mast last year. In addition to the halyard hook rivets, the head cap was also glued in so removing was a bit of a project (and I ended up cracking the comptip, so be careful http://www.hobiecat.com/forums/viewtopic.php?f=20&t=44683&p=189552&hilit=comptip#p189552
). Just below the head cap, there was a thin fiberglass "plug" which I also had to destroy and remove to get the water out. In retrospect, drilling a small drain hole just below the halyard hook and then plugging with a rivet may have been the better route.