Has your engine been over heating? Was it the coolant in the expansion tank that looked like mud, or the coolant in the radiator itself? Does the engine run poorly?
Four quarts of oil is too much for the engine to have to have added. The engine must be leaking oil internally, externally, or "burning" oil. It is usually water that mixes with the oil rather than oil mixing with the coolant when the head gasket blows. It sounds like your car is not drivable and needs to be checked out by a mechanic as soon as possible but in the meantime do not drive it have it towed as driving it further may do expensive damage.