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?
When I say "technically," I mean this is what the dictionary says. So if you are taking some Mandarin oral exam (presumably a strict one), yes, you must say gao4su0 to mean "inform." It's not a literary or colloquial choice; it is the only definitive pronunciation according to the dictionary.
You may conveniently take as a general rule that "the light tone sounds less formal", though it's not an ironclad rule. For instance, 玫瑰[rose], a neutral, proper noun without any contextual indication, is pronounced mei2gui0. The light tone here has nothing to do with informality, it is just the definitive pronunciation in the dictionary.
Bear in mind the third rule posted by SuperXW: The forth tone is close to the light tone and sometimes even a native speaker cannot distinguish them. This explains why most native speakers are not aware that gao4su0 and gao4su4 are actually different two words, since they sound pretty much the same. In other words, it doesn't really matter in daily life if you pronounce the forth tone instead of the light tone, as it doesn't change much to one's ears and is totally understandable.