My partner and I play that a 3 NT response after any intervening overcall of one NT promises sufficient values for game, denies a 4-card Major suit, and also denies  having a stopper in the suit bid by the opponent  ("direct denies"). 

The opening NT bidder can do one of three things: (1) Pass with a stopper in the suit ops bid.  (2) Bid a five card major if he/she has one or (3) explores for a better contract, generally in one of the minor suits.

So with your hand, I will bid four spades showing a five card major suit.