I've tried to make yougurt but I've got soured milk - what went wrong?
Either you failed to adequately pasteurize the milk, or your culture was bad, or some unclean / unsanitized (some would say sterilized, I know better) utensil was involved on the cooling/inoculating side of the pasteurization step.

4 hours is a remarkably short incubation time - were you aiming for a "liquid/drinkable" yogurt? For a set yogurt 8-12 hours is typical and my process at this point is 24 hours at a somewhat lower temperature (95-100F) as I like the result better than the warmer/shorter incubation.