Our system is based on bidding 4-card suits up the line, rather than giving preference to a 4-card major.
A natural consequence of this is that a 1♠ opening is almost always based on a minimum 5-card suit, which is why with many partners we have the agreement that it guarantees a minimum 5-card suit, a 4-3-3-3 hand outside the 1NT range being opened 1♣.
Without such an agreement, if opener has only a 4-card spade suit, he must be outside the 1NT range, i.e. 15+ and probably 16+ [this latter because 4-3-3-3 hands are relatively weak distributionally, so such a 15-count hand may well be opened 1NT anyway].