I'm not sure about the exact number, but doesn't a Saiyan's power level quadruple when they turn into an ape, compared to SSJ3? I think I read it as either SSJ3 * 4 or SSJ4 * 4 at some official unofficial Dragonball site.
I don't really recognize DBGT (where the first occurrence of SSJ4 happened) as a true Dragonball series, though, since the original author of the DB and DBZ series had no part in making it and from what others say, it's not as good as DB or DBZ was.
But I've never been able to understand a DBGT episode, since I don't speak Japanese.