Context. We present new spectroscopic and polarimetric observations of the gravitational lens SDSS J1004+4112 taken with the 6 m telescope of the Special Astrophysical Observatory (Russia).
Aims: In order to explain the variability that is observed only in the blue wing of the C IV emission line, corresponding to image A, we analyze the spectroscopy and polarimetry of the four images of the lensed system.
Methods: Spectra of the four images were taken in 2007, 2008, and 2018, and polarization was measured in the period 2014-2017. Additionally, we modeled the microlensing effect in the polarized light, assuming that the source of polarization is the equatorial scattering in the inner part of the torus.
Results: We find that a blue enhancement in the C IV line wings affects component A in all three epochs. We also find that the UV continuum of component D was amplified in the period 2007-2008, and that the red wings of CIII] and C IV appear brighter in D than in the other three components. We report significant changes in the polarization parameters of image D, which can be explained by microlensing. Our simulations of microlensing of an equatorial scattering region in the dusty torus can qualitatively explain the observed changes in the polarization degree and angle of image D. We do not detect significant variability in the polarization parameters of the other images (A, B, and C), although the averaged values of the polarization degree and angle are different for the different images.
Conclusions: Microlensing of a broad line region model including a compact outflowing component can qualitatively explain the C IV blue wing enhancement (and variation) in component A. However, to confirmed this hypothesis, we need additional spectroscopic observation in future.

The reduced spectra are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/634/A27