Essentially nil; like many other ancient Greeks, he was talking out his hat and coincidentally happened to come up with something that's not entirely unrelated to what we now know to be true.
In the case of Empedocles, it was the suggestion that all matter might be made of combinations of a few types of basic substances. He got the number wrong, he got the substances wrong, he got the forces wrong, and he tried to work in mythology somehow, but in a sense it's the basis for the notion of elements (though he didn't use that term).