Context : Dome C in Antarctica is a promising site for photometric observations thanks to the continuous night during the Antarctic winter and favorable weather conditions .

Aims : We developed instruments to assess the quality of this site for photometry in the visible and to detect and characterize variable objects through the Antarctic Search for Transiting ExoPlanets ( ASTEP ) project .

Methods : We present the full analysis of four winters of data collected with ASTEP South , a 10 cm refractor pointing continuously toward the celestial south pole .
We improved the instrument over the years and developed specific data reduction methods .

Results : We achieved nearly continuous observations over the winters .
We measure an average sky background of 20 mag arcsec ^ { -2 } in the 579–642 nm bandpass .
We built the lightcurves of 6000 stars and developed a model to infer the photometric quality of Dome C from the lightcurves themselves .
The weather is photometric 67.1 \pm 4.2 % of the time and veiled 21.8 \pm 2.0 % of the time .
The remaining time corresponds to poor quality data or winter storms .
We analyzed the lightcurves of $ σ $ ~Oct and HD~184465 and find that the amplitude of their main frequency varies by a factor of 3.5 and 6.7 over the four years , respectively .
We also identify 34 new variable stars and eight new eclipsing binaries with periods ranging from 0.17 to 81 days .

Conclusions : The phase coverage that we achieved with ASTEP South is exceptional for a ground-based instrument and the data quality enables the detection and study of variable objects .
These results demonstrate the high quality of Dome C for photometry in the visible and for time series observations in general .