Most of the time in this style of playing, you are mostly using your 1st, 2nd, and 3rd fingers. If you were to use your 4th finger, you would have to move back into the home base position. For more information on pentatonic scales check out these lessons. All of the basic improvisation lessons deal with pentatonic scales. Turn your hand to the side, and even rest the outside of your 1st finger on the bottom of the neck.
Pentatonic position is also lends itself well to bends .
When you bend a string, you need to have the support of having your thumb up on top of the neck. Here are a couple of videos to check out of me playing in pentatonic position. No matter what position you left hand is in, avoid putting the palm of your hand on the back of the guitar. Your thumb might be wrapped over the top of the neck, but there will still be space between you palm and the back of the neck.