PSR B1620-26, in the globular cluster M4, is a millisecond pulsar with a ∼0.3 M ○. companion in a ∼0.7 AU orbit. It was recently realized (Backer 1993) that timing observations of this pulsar show, in addition to a linear spin-down, a large-period second derivative naturally interpreted as evidence for a varying acceleration, or jerk, of the pulsar binary. We describe timing observations of PSR B 1620-26 taken over a 5 year period. The measured acceleration and jerk are inconsistent with acceleration by the mean gravitational field of M4, and an encounter with another star close enough to cause the observed acceleration is very unlikely