Blasting 4N doesn't make sense, because If we get answer of 1 or 4, then we're none the wiser about partner's hand. Of course, with a 5♥ or 5♠ response, we might be able to roll it in with a finesse in trumps. I think X is probably the best action, so we can now workout exactly where we want to be. If they bid spades a 4-3 fit might be playable. X keeps our options open,whereas any forcing 4N, 4H, 4C could just get messy.