We present the design and experimental demonstration of an ultrathin four-reflection imager. The $F/1.15$ prototype imager achieves a focal length of $18.6\mathrm{mm}$ in a track length of just $5.5\mathrm{mm}$ , providing a $17°$ field of view over 1.92 megapixels of a color image sensor with $3\mathrm{\mu m}$ pixels. We also present the design and experimental results of pupil-phase encoding and postprocessing, which were applied to extend the depth of field and compensate a small amount of axial chromatic aberration present in the four-reflection imager prototype.