4NT can mean a number of things, depending on the situation.
- As a jump bid, after partner's last natural bid was a suit, 4NT is typically Blackwood.
- As a jump bid, after partner's last natural bid was notrump, 4NT is typically a quantitative slam invite.
- As a nonjump bid, 4NT may simply be a signoff.
- In response to Gerber 4♣, 4NT shows 3 aces.
- As an opening, 4NT is usually a form of ace asking bid, e.g. Blackwood.
- As an overcall, it typically shows a two-suiter (unusual notrump overcall).