The 4th, 5th and 6th strings on the guitar are referred to as the 4th set.
All of the triads on the execises below use the 4th set only.
Root position triads
1st Inversions
2nd Inversions
In the following exercise the voicings of each triad are played starting with the voicing closest to the nut of the guitar and ending with the one furthest from the nut.