It's a Dsus4 chord. It's called this because of the intervallic distance between d and g which is a 4th, instead of g to d, which is a power-chord or G5. Even though it's the same notes the backwards relationship makes it sound like a suspension. Power-chords don't have inversions because it's function will change. The next logical move is to move the G up a whole step or down a half step.