sus4 does not mean there are four notes.
It means the 3rd is replaced by the 4th.
In jazz, the fuller sound is usually:
1 4 5 b7
On C7sus4:
So here, 1 4 5 b7 means:
- 1
- 4
- 5
- b 7
Compared with Cmaj7, the 3 disappears and the 4 takes its place. Because it is a dominant-type sound, the 7 is lowered.
Because the chord avoids the 3rd, it feels more open and less settled than a plain major or dominant chord.