You can safely uninstall the older versions. If you need to reinstall the newest, you will only need the old serial numbers. Old versions do not actually have to be installed for an upgrade.
No, you don't need to have a previous version installed to run an upgrade version. That's not how upgrades work. Upgrades have nothing to do with the software; upgrades are just a price break on licensing.
You can uninstall Premiere Pro CS4 without problems. But why? Hard disk space is cheap, and you may need the old version at some point. I have CS3, CS4, CS5, and CS5.5 installed on the same machines.
Looks like Jim types faster than I do. ;-)
Let me give you a warning about this.
I was having problems that cropped up coincident with the upgrade Pr CS5.5.1, and I decided to revert to CS5.5.
I ran the suite uninstaller app, and UNchecked ONLY Premiere Pro, but the installer wiped out ALL my CS5.5 apps.
There is a "Uninstall Premier Pro" aliais in the Premiere Pro CS5.5 application folder. Maybe I should have run that instead. Anyway... word to the wise if anybody else gives this a try.
BTW, the uninstaller did not wipe out several plugs for AE in the MediaCore folder. That was kind of lucky for me, because otherwise I could have had to spend an hour or so reinstalling and authorizing them.* But, OTOH, the Adobe uninstallers kind of suck, because they don't remove everything. They never did. I've always had vestiges of previous installs on my drive after running their uninstaller scripts. In this case, it worked to my benefit. So, as I say, mixed blessing, I suppose.
* One more bit of advice: I keep a clone of my boot drive, updating it daily on most occasions. It's an easy matter to reinstall components from another hard drive rather than go through a time-consuming re-install process. Having a clone means I don't have to go through the arduous re-install and re-authorize process that "could have" been the case without the clone; just copy the files over at hard drive speeds.