Watch the Double Stops online guitar lesson by Brad Carlton from Guitar Lab: Big Blues & Beyond
This lesson covers double-stop licks in which the 4 is the lower voice. The sound produced with this technique will remind you of B3 organ style playing.
We'll begin with the 4 of the C blues scale 6th string root form in the eighth fret of the 5th string. This will be the lower voice. Then you'll play the C blues scale that exists above it on strings 4 through 1.
I'll show you how to use this information to outline the IV chord.