I heard he uses 2 A's for the E and the A, and uses 2 G's for the G and the D.....I'm not sure tho, so I'd get a second oopinion if I were you.
http://mama.indstate.edu/users/primus/html/articles.html In one of the interviews he says specifically what he plays but I know for sure he uses 2 A strings and 2 G strings, he says it makes chords sound better.
I was under the impression that he only uses those string gauges for a couple of songs, most notably "Is it Luck?" So I guess he strings one of his 4 string basses like that, but for his other basses? You think he strings his Rainbow bass like that? Oog
I thought on the CT website it says he uses extra-light CT strings. I've heard the 2xA and 2xG thing, though.