If you install it under a different directory than qt3, you should not have any problem. Have a look at docs.
Oh, I saw a little message about a pre kde 2 version (have a look at apps.kde.com). It will use qt2, maybe that is already installed on your machine.
As I'm curious: Is there a special reason you want using caitoo, 'cause I think there are other download managers, which I think will be easier to installed.