On Wed, 9 Nov 2011 12:10:19 -0800, Bob E. wrote:

What does U4 do?

Provides a Vcc/2 local "ground" so I can use these op amps with a single
supply voltage.

Hint: Don't use the chassis ground symbol as a "Vref" symbol. It's confusing
and someone along the line might get hurt. Grounds should be.