Context : After years of modest optical activity , the quasar-type blazar 4C 38.41 ( B3 1633+382 ) experienced a large outburst in 2011 , which was detected throughout the entire electromagnetic spectrum , renewing interest in this source .

Aims : We present the results of low-energy multifrequency monitoring by the GASP project of the WEBT consortium and collaborators , as well as those of spectropolarimetric/spectrophotometric monitoring at the Steward Observatory .
We also analyse high-energy observations of the Swift and Fermi satellites .
This combined study aims to provide insights into the source broad-band emission and variability properties .

Methods : We assemble optical , near-infrared , millimetre , and radio light curves and investigate their features and correlations .
In the optical , we also analyse the spectroscopic and polarimetric properties of the source .
We then compare the low-energy emission behaviour with that at high energies .

Results : In the optical–UV band , several results indicate that there is a contribution from a quasi-stellar-object ( QSO ) like emission component , in addition to both variable and polarised jet emission .
In the optical , the source is redder-when-brighter , at least for R \ga 16 .
The optical spectra display broad emission lines , whose flux is constant in time .
The observed degree of polarisation increases with flux and is higher in the red than the blue .
The spectral energy distribution reveals a bump peaking around the U band .
The unpolarised emission component is likely thermal radiation from the accretion disc that dilutes the jet polarisation .
We estimate its brightness to be R _ { QSO } \sim 17.85 –18 and derive the intrinsic jet polarisation degree .
We find no clear correlation between the optical and radio light curves , while the correlation between the optical and \gamma -ray flux apparently fades in time , likely because of an increasing optical to \gamma -ray flux ratio .

Conclusions : As suggested for other blazars , the long-term variability of 4C 38.41 can be interpreted in terms of an inhomogeneous bent jet , where different emitting regions can change their alignment with respect to the line of sight , leading to variations in the Doppler factor \delta .
Under the hypothesis that in the period 2008–2011 all the \gamma -ray and optical variability on a one-week timescale were due to changes in \delta , this would range between \sim 7 and \sim 21 .
If the variability were caused by changes in the viewing angle \theta only , then \theta would go from \sim 2.6 \degr to \sim 5 \degr .
Variations in the viewing angle would also account for the dependence of the polarisation degree on the source brightness in the framework of a shock-in-jet model .