Watch the Fourths online guitar lesson by Robbie Calvo from Double Stop Chops
I'm going to strongly advocate that you learn a few key in position places in which to employ your double stop 4ths and not worry about learning string length lines. Why? Because I believe that moving across the fretboard with 4ths actually sounds better and is what you've come to know when you hear it. If you play 4ths too linearly, they have a tendency to sound oriental or non-R&B in flavor. Let's take a look at some positions that I think are imperative to making nice lines from 4ths.