Watch the Four Voice Frenzy online guitar lesson by Brad Carlton from Chord Studies: CAGED Chords Vol. 3
This is a I - V - vi - IV progression in the key of D. The first chord is a D chord using a C form out of the CAGED system. Our next chord is A/C♯, which is the V chord with the 3rd in the bass, utilizing the G form. Next we move to a Bm using the Am form, and then a G chord coming out of the E form. We are using four voicings throughout on strings two through five.