Apart from suit bids opposite 3 and 3, all neutral bids above 4 are natural if they can be, the suit bids being suit-setters and the game bids and slam bids being to play.

Neutral 4NT is a slam invitation if a jump, but otherwise a sign-off.

WestNorthEastSouth
1pass2pass
2pass4NTpass
?
Bid some slam with a maximum, otherwise pass.
WestNorthEastSouth
1NTpass4pass
4NTpasspasspass
Both 4 and 4NT are to play. Presumably this is pairs, or opener thinks he plays dummy better than responder.

Neutral 5 and 5 are Italian fit bids, described in fit auctions.

NEXT: neutral opening bids