Agree with stephen. Technically, gao4su0 and gao4su4 are different. The latter is a formal term, and is much more used in Taiwan than in the Mainland. SuperXW offers a general rule, but I'm afraid it is not the case here.
So you mean that gàosù is wrong if I want to say "to inform"? I MUST say gàosu, even if I am using literary Chinese?