I actually learned blues lead with this tip as a foundation. An interesting thing to note about this suggestion is that your 5th string root is the 6th string root of the V. In other words, the box 4 shape of the root, overlays the box 1 shape of the V. Or, said yet another way is that the box 1 shape over the 12th fret is E, which is the V of A. So, when someone asks "when do licks in the box 4 shape sound good?" The answer is when you hit the V chord of the progression. The reason is because if you look at the two boxes overlayed, you'll see they share a lot of the same notes in the pentatonic...with the added benefit that in box 4 over the V you can hit/bend to notes in the V that only sonically work over the V. So, instead of just transcribing your licks from Box 1 to Box 4, you can alter them to hit chord V notes that only work in that particular chord of the progression. I hope that makes sense. Because once you get your head wrapped around that, you'll see that you can also overlay the box 4 shape over the box 1 shape of the A/tonic to get the D/IV chord. And, similarly you can shift up from the 5th fret to the 7th fret and use the box 4 shape there for a D/IV pentatonic shape. For me, that means I can take the 10-20 licks I know in Box 1 and 4 of any key, and nearly double my repertoire if I modify them for IV/V chords.
