  Spectral state transitions in neutron star LMXB systems have been widely
observed yet not well understood. Here we report an abrupt spectral change in
4U 1608-52, a typical atoll source, during its decay phase of the 2004
outburst. The source is found to undergo sudden changes in its spectral
hardness and other properties. The transition occurred when its luminosity is
between (3.3-5.3) E36 ergs/s, assuming a distance of 3.6 kpc. Interpreting this
event in terms of the propeller effect, we infer the neutron star surface
magnetic field as (1.4-1.8) E8 Gauss. We also briefly discuss similarities and
differences between the spectral states of neutron star and black hole binary
systems.
