Most step motors have a shallow flange (round) inside the square flange. Some mounting brackets if machined to close tolerances can "capture" the flange. If you romoved the four mounting bolts and it's still not loose then take a rubber mallet and GENTLY tap around first the flange and then the motor body. Depending on how long the motor has been mounted it may have stuck itself to the mounting flange. There is no hidden fasteners or set screws. The motor shaft does not slide out (if it does you can throw the motor in the trash)
TOM caudle
www.CandCNC.com