I do not agree with the defensive intentions of the double. I would rather play the double as negative. As if overcaller had bid 2!h and not 2!s. So the double means also !s support, usually a 3 card and some 8 points or stronger. This meaning of the double influences the meaning of 4 spades of course. 4 spades means: I like to play 4 spades opposite a normal double, I have no defensive intentions against 3 hearts and no slam intentions opposite an average double.