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 upper voice. The sound produced with this technique will remind you of B3 organ style playing.
We'll actually go outside the 6th string root form of the C blues scale for our highest upper voice. The 4 is available in the thirteenth fret of the first string. You'll then play the C blues scale that exists below it in eighth position on strings 2 through 6.
You can also use the 4 which occurs in the tenth fret of the 3rd string as the upper voice, then play the C blues scale below it on strings 4 through 6.
I'll show you how to use this information to outline the IV chord.