The difference between sus (suspended) and add4 chords may not be what you think.
A sus chord contains the 1st, 4th, and 5th notes of the major scale. It's important to note that a sus chord does not contain a 3rd.
An add4 chord contains the 1st, 3rd, 4th, and 5th notes of the major scale. It's important to note that an add4 chord contains both a 3rd and 4th.
Gsus
The Gsus chord contains just 3 notes: G, C, and D. The 1st, 4th, and 5th notes of the G major scale.