  We present polarization and Faraday rotation for the supernova remnants
(SNRs) G46.8-0.3, G43.3-0.2, G41.1-0.3, and G39.2-0.3 in L-band (1-2 GHz) radio
continuum in The HI/OH/Recombination line (THOR) survey. We detect polarization
from G46.8-0.3, G43.3-0.2 and G39.2-0.3 but find upper limits at the 1% level
of Stokes I for G41.1-0.3. For G46.8-0.3 and G39.2-0.3 the fractional
polarization varies on small scales from 1% to ~6%. G43.3-0.2 is less polarized
with fractional polarization <~3%. We find upper limits at the 1% level for the
brighter regions in each SNR with no evidence for associated enhanced Faraday
depolarization. We observe significant variation in Faraday depth and
fractional polarization on angular scales down to the resolution limit of 16".
Approximately 6% of our polarization detections from G46.8-0.3 and G39.2-0.3
exhibit two-component Faraday rotation and 14% of polarization detections in
G43.3-0.2 are multi-component. For G39.2-0.3 we find a bimodal Faraday depth
distribution with a narrow peak and a broad peak for all polarization
detections as well as for the subset with two-component Faraday rotation. We
identify the narrow peak with the front side of the SNR and the broad peak with
the back side. Similarly, we interpret the observed Faraday depth distribution
of G46.8-0.3 as a superposition of the distributions from the front side and
the back side. We interpret our results as evidence for a partially filled
shell with small-scale magnetic field structure and internal Faraday rotation.
