Watch the Swingin' IV online guitar lesson by Jeff McErlain from Solo Factory: Chicago Blues
This swingin' IV chord lick is easiest to see as an Emajor blues scale that avoids playing the G#. It can also be seen as coming from the A mixolydian scale as it is which is a little more difficult to thinks about. I always try to simplify these things down to the basics. In this lick we are really outlining just the chord, which is part of the Chicago blues sound and it's jazzy roots.