"D4" or "D-4" can mean:
*D4 video connector
*A polyhedral dice commonly referred to as a "d4", see 4-sided die
*A Dublin postcode see Dublin 4
*D4 framing standard
*The D4, a rock band
* D4, a programming language of the Dataphor system
*A square on a chess board, as well as a move or even opening pertaining to that square.
*Driver: Parallel Lines, the fourth videogame in the Driver series
*Alidaunia IATA code
*A very annoying posh person from Dublin 4 who wears dubes, and is often seen saying such words as 'roysh', 'rooogby' and 'omigod!'