  Four pulsars, PSRs J1838+1523, J1901+0510, J1909+0007 and J1929+1844, are
found to exhibit bright and weak emission states from sensitive FAST
observations. New FAST observations have measured their polarization properties
for the two states, and revealed that the polarization profiles, linear
polarization percentage, and polarization position angle curves, as well as
circular polarization percentage are partially or entirely different in the two
emission states. Remarkably, PSR J1838+1523 has very different slopes for the
polarization position angle curves. PSR J1901+0510 has a wider profile and a
higher linear polarization in the weak state than those in the bright state.
PSR J1909+0007 has very distinct polarization angle curves for the two modes.
While in the case of PSR J1929+1844, the central profile component evolves with
frequency in the bright state, and the senses of circular polarization are
opposite in the two modes. The different polarization properties of the two
emission states provide valuable insights into the physical processes and
emission conditions in pulsar magnetosphere.
