First , the G note is on the 3rd fret not the 4th on the first string... And I think you mean the 6th string by saying the first string , but it makes no difference in this case since they're both Es.

A power chord is just the root note and the 5th , the root note is the name of the chord ... So the chords you need are :

G5 (power chord) = G + D
D5 = D + A
C5 = C + G


Generaly , add 3.5 steps (or 7 frets) to the rrot note , and you got the power chord... You can also add the octave note to ave a 3-string power chords which increase the thickness of the chord.