  We observed three radio relics in galaxy clusters and one radio relic
candidate at 4.85 and 8.35 GHz in total emission and linearly polarized
emission with the Effelsberg 100-m telescope and one radio relic candidate in
X-rays with the Chandra telescope. The radio spectra of the integrated emission
below 8.35 GHz can be well fitted by single power laws for all four relics. The
flat spectra (spectral indices of 0.9 and 1.0) for the "Sausage" relic in
cluster CIZA J2242+53 and the "Toothbrush" relic in cluster 1RXS 06+42 indicate
that models describing the origin of relics have to include effects beyond the
assumptions of diffuse shock acceleration. The spectra of the radio relics in
ZwCl 0008+52 and in Abell 1612 are steep, as expected from weak shocks (Mach
number $\approx 2.4$). We find polarization degrees of more than 50 % in the
two prominent Mpc-sized radio relics, the Sausage and the Toothbrush. The high
degree of polarization indicates that the magnetic field vectors are almost
perfectly aligned along the relic structure. The polarization degrees
correspond to Mach numbers of $>2.2$. Polarized emission is also detected in
the radio relics in ZwCl 0008+52 and, for the first time, in Abell 1612. Abell
1612 shows a complex X-ray surface brightness distribution, indicating a recent
major merger. No Faraday depolarization is detected between 4.85 GHz and 8.35
GHz, except for one component of the Toothbrush relic. Faraday depolarization
between 1.38 GHz and 8.35 GHz varies with distance from the center of the host
cluster 1RXS 06+42, which can be explained by a decrease in electron density
and/or in strength of a turbulent magnetic field. Faraday rotation measures
show large-scale gradients along the relics, which cannot be explained by
variations in the Milky Way foreground. Large-scale regular fields appear to be
present in intergalactic space around galaxy clusters.
