Obviously there are many bays out there much shorter, such as the Raven for 38's....only 2 inch. I have lost 2 rockets due to shearing off the top of my fin can at Max-Q using such a short coupler.
You must fit the proper length coupler to your project, based on maximum thrust your largest motor you intend to use, & stresses it can create on your airframe. This will also depend on WHAT the coupler & what the airframe, is made out of...i.e. paper, phenolic, glass, carbon, metal.
It is also imperative you have a very tight, smooth fit for high power, high stress flights or bad things will happen.:dark:
All that being said, if just sport flying high-power, one can "get away" with shorter couplers.