  We present slight refinements of known general lower and upper bounds on
sizes of extended formulations for polytopes. With these observations we are
able to compute the extension complexities of all 0/1-polytopes up to dimension
4. We provide a complete list of our results including geometric constructions
of minimum size extensions for all considered polytopes. Furthermore, we show
that all of these extensions have strong properties. In particular, one of our
computational results is that every 0/1-polytope up to dimension 4 has a
minimum size extension that is also a 0/1-polytope.
