Adding a plus button requires that the tab area always be visible. This might be fine if you're not concerned with screen real-estate, but otherwise it should be avoided. Of course, if the tab area is already visible because at least two tabs are open, then having a plus button might be fine. Of course, that takes up horizontal real-estate that might be better occupied by tabs and the useful information they display!
I do like Solution #4, however, which is precisely what KDE 4.4 has. And the tabs are placed in the window border/decoration so no screen real-estate is lost (except for the horizontal space!)