We report the detection of four images in the recently discovered lensed QSO RX J0911.4+0551. With a maximum angular separation of 3 ".1, it is the quadruply imaged QSO with the widest known angular separation. Raw and deconvolved data reveal an elongated lens galaxy. The observed reddening in at least two of the four QSO images suggests differential extinction by this lensing galaxy. We show that both an ellipticity of the galaxy (epsilon(min) = 0.075) and an external shear (gamma(min) = 0.15) from a nearby mass have to be included in the lensing potential in order to reproduce the complex geometry observed in RX J0911.4+0551. A possible galaxy cluster is detected about 38 " from RX J0911.4+0551 and could contribute to the X-ray emission observed by ROSAT in this field. The color of these galaxies indicates a plausible redshift in the range of 0.6-0.8.