Ventrilo is far better than Roger Wilco or Teamspeak and I can recommend it to anyone
A lot of programs have difficulties with mouse4 or anything driver specific like that.
I suggest remapping the mouse4 to a button like pagedown (or another one you never use in a game), that will make sure there's no conflicts between the programs, as long as you unbind pagedown from ut2k4 and db
I'm sure your drivers will allow that, if you have Logitech, I'm sure they do
I've got my thumb buttons bound to pageup and pagedown, and my 8th button bound to o (I don't use that in-game, so that's why, can be anything)
Pageup is "Use Boost" in DB, and bound to the handgun in ET
Pagedown is "Pass to Me!" in DB, and bound to SMG in ET
The 8th 'application switch'(alt+tab ripoff) button is bound to o, which is 3rd special weapon in ET, and bound to adminlogin for gameservers.net servers in DB.