I would bet it is probably going to be a while before we see Vegas 5. I'm sure everything is still getting straightened out with the Sony deal, and then it will probably take a while for them to figure out exactly where they want to go with the Vegas product.
I have absolutely ZERO insider information, just a wild guess. I would look for Vegas 5 about 15-18 months from the release date of Version 4. That seems to be a fairly typical pattern for "successful" software.
So lets do the math. Vegas 4, DVD-A was released early 2/2003, if I remember correctly, so I would start expecting a new version somewhere in the early spring to mid summer range of next year.
I doubt the Sony purchase will have any major impact on the time released at all.
Posters: Did you notice that SoFo rushed in, cape flying, barely 90 minutes after the post to ask what the trouble is? With support like that we don't need no stinkin' versions!