Sus4 chords replace the 3rd note of the scale with the 4th: 1, 4, 5
This gives an open, suspended sound as we expect to hear the sus4 chord resolve to a major chord - hence the name suspended 4th.
You don't have to see sus4 chords written down to play them. Anytime you see a major chord, you can try embellishing it with a sus4 chord before playing the major version.
Check out other sus4 guitar chords and tune back soon for another guitar chord of the day.