A power chord is just two notes: the root and the fifth. No major, no minor - it works over anything. In fourths tuning the shape is the same everywhere: root on any string, fifth two frets higher on the next string.
Why fret + 2? Each string in P4 tuning is 5 semitones apart. A perfect fifth is 7 semitones. So you need 2 extra frets on the next string to make up the difference (5 + 2 = 7). One shape, every key, every string pair.
The Shape
Here are power chords on the low E string. The root moves up the neck and the shape stays identical. Tap any chord to hear it.
Move It to Any String
The same shape works on every string pair. Here are D power chords starting from three different strings. The interval is always root + 2 frets on the next string.
Add the Octave
For a fatter sound, add the octave on the third string. Two strings up from the root is 10 semitones (5 + 5). An octave is 12 semitones. So the octave sits at root fret + 2 on the third string. This gives you a 3-string power chord.
Add the 5th Below
You can also add the 5th on the string below the root for an even heavier sound. Instead of root-5th, you play 5th-root-5th - the low 5th adds depth and weight. This shape is all over metal and hard rock. Death used it constantly - listen to Crystal Mountainfor a masterclass in how this voicing drives a riff.
In P4 tuning this is particularly clean: the 5th below the root is always on the same fret as the root, one string lower. So A5 with the low 5th is frets 5-5-7: same fret for the E (5th) and A (root), then the high E two frets up on the D string.
Practice
Play E5, A5, D5, G5 as a simple progression. Use all downstrokes, one strum per beat at 80 BPM. Focus on muting the strings you are not playing by letting your fretting finger lightly touch them. Once comfortable, try the same progression starting on different strings. Then try the 5th-below shapes - play the riff from Crystal Mountain as a starting point.