Watch the Example 19 online guitar lesson by Brad Carlton from Guitar Lab: Boogie-Woogie

This is a great jazzy passage out of A mixolydian. The chords are Em/A (A9 no 3rd), A6, G/A (A9sus4) and A6. This is another example of the 3 to 4 move (A6 to A9sus4) which blurs the "3 side" and "sus4 side" of the dominant family of chords.