In theory... a 4 rotor can be built from 13B housings, centreplates and endplates with a new, custom eccentric shaft... and a new Crank Angle Sensor to run it... and new, custom manifolds.... and all the other new, custom stuff I'm forgetting...
This is another "gunna" spoken about in pubs and workshops worldwide. I'm gunna make a 4rotor out of those 13B bits in the corner one day.... It'll be fullllly siiiik maaate...
It would be more realistic to make a 20B (3 rotor) using this principle (with a 20B shaft and CAS) but then again why wouldn't you just by a 20B motor to begin with...