I would rather bid 4S than 1N, but anyway if I did, it seems percentage to just bid 4S over a 2C or 2H rebid by partner, but if partner bids 2D I would only bid 3S. Also if you are wondering why I think it is percentage to bid 4S instead of 1N even though opposite some rebids I wouldn't force to game (that is, 1N is more "accurate"): 1) 1M-4M is a great auction for your side and is very hard on the opponents, both in the bidding and the play. Giving away information about partner's hand with his rebid is very bad when we are probably just going to play a (light) 4S contract. 2) 3 card and 4 card limit raises are different, partner will not evaluate his hand correctly when he has bad trumps (among other things), which is not unlikely given our spade holding.