A switchable microstrip quad-band bandpass filter (BPS) is investigated in this study. Owing to the low loading effect from distributed coupling technique, many channels can be obtained without the requiring of complicated matching network. Each passband can be turned on and off independently. The switchable quad-band BPS consisting of half-wavelength/quarterwavelength resonators, loaded open stubs and p-i-n diodes features compactness and good selectivity. To validate this design concept, theory as well as experiment is presented and compared in this study.