What is D4 chord?

The D4 chord, often called a D4sus (suspended) chord, is used in piano music to create a fuller, more interesting sound in certain songs. The “4” in the chord suggests the presence of a note that is a fourth above D, which happens to be the G note.

What note is D4?

In pitch notation, the notes C4, D4, and B4 belong to the same pitch class, which is the fourth octave.