Relativistic phenomena (orbital precession, Shapiro delay, and/or orbital decay) have been measured in Arecibo timing observations of four pulsar-white dwarf binaries, leading to constraints on the neutron star masses. We have detected the decay of the PSR J0751+1807 orbit due to gravitational radiation emission, the first such measurement in a binary with a low mass ratio (m
1 ≪ 1). Timing data constrains the mass of this pulsar to bet between 1.6 and 2.8 M⊙. Masses of the other pulsars are in marginal agreement with the canonical pulsar mass of 1.35 M⊙, but higher values are preferred.