Hubble Space Telescope astrometry of M4 and the Galactic constant V0/R0

From multiepoch WFPC2/HST observations we present astrometric measurements of stars in the Galactic globular cluster M4 (NGC 6121) and in the foreground and background. The presence of an extragalactic point source allows us to determine the absolute proper motion of the cluster and, through use of the field stars in this region only 18degrees from the Galactic center, to measure the difference between the Oort constants, A - B. We find (mu(alpha) cos delta, mu(delta))(J2000.0) = (-13.21 +/- 0.35, -19.28 +/- 0.35) mas yr(-1), and A - B = V-0/R-0 = 27.6 +/- 1.7 km s(-1) kpc(-1).