About to sit a mock history GCSE tomorrow and it's pretty vital info. I'd been told Hippocrates for a year, then the teachers changed their minds and said it was Aristotle. But Hippocrates died when Aristotle was 14, so who actually was it? Thanks :)
The theory of the four humours is ancient and the exact origin is unclear, but the root of the Greek theory is generally accepted to have come from a text called The Hippocratic Caucus, which might not have been written by Hippocrates himself but certainly came from his school.
It's worth noting that the theory was finalised in Western medicine by the writings of Galen in the second century, who would act as one of the most prominent medical authorities all the way until the sixteenth century.
That said, I remember GCSE History. If in doubt go with what's in the syllabus and study guides, that's what they'll be looking for.
Hippocrates is typically credited with the four humors though he probably didn't invent it from scratch. Aristotle took Hippocrates' notion about the body and integrated into his general physical cosmology, so each element corresponded to a humor. Galen (much later — Roman times) is who made the theory much more known to the Islamic world and eventually that is what led to it being known in the world of medieval Europe, through people like Avicenna. Just to flesh it out a bit.