Watch the Drop 2, 3, 4 Voicings online guitar lesson by Brad Carlton from Guitar Lab: The Golden Arp
The final inversion of the drop 2, 3, 4 voicing of the add 9 chord is 3rd inversion. There are four layouts on the fingerboard. The 5 is the lowest voice, so a power chord is produced when playing over a root bass note. The sound will be fat and full and the remaining 1 2 and 3 voices will fill out the sound.