power chords are derived from bar chords, the 4,5,6 you are talking about comes from the E style bar chord so 6th string 3rd fret and 5th and 4th string fifth fret would be a G powerchord.

The same you can do with the A style bar chord so 5th string 3rd fret and 4th and 3rd string fifth fret would be a C powerchord.

The same apllies with a little bit of different fingering for the D style bar chord so 4th string 2nd fret, 3rd string fifth fret and here is the difference; 2nd string [u]6th fret [/u]would be an E powerchord due to the differnt tuning of the G string.

Hope this helps

Herman