Dsus4

D1G4A5

The third replaced by the fourth, which leans on the third it is standing in for.

What is true of it

Dsus4 is D, G and A — 1, 4 and 5 counted from D.

These are also the notes of Gsus2/D. Which name is right depends on what is underneath and what came before — the notes alone do not decide.