Yes, that would be a problem uninstalling 4 and trying to use just 5.5. Like you said, it's an upgrade, not a full version, so you can't use 5.5 unless you have 4 already installed.
Ummmm.....It's been a while, but I do believe that there was a version 5.0 that was realeased. Where did that go? I'd call/email Adobe support and ask them if you'll run into conflicts jumping from 4 to 5.5 (I believe that 5.5 was a major update for photoshop version 5).
IMHO, you might just be better off going to version 6.0, then updating to v 6.0.1. It's very stable, and I've yet to have any major problems with it.....ever. I've been using it for a few years now.
As far as your plugin issue, I'm not quite sure about that. I'd try uninstalling all versions of photoshop that you have now, restart, then reinstall them again. Be sure to update each individual version before instatlling the newer version (update 4 then restart, then install 5.5, then upgrade 5.5 then restart).
After you uninstall everything, restart your computer and run Ccleaner whcih you can download for free HERE
before you start to reinstall everything. This should delete anything left behind by photoshop.
Hope all this helps!
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