The cover's flange is typically stuck to the mating flange on the magnet ring with sealant making it difficult to separate the two parts.
I usually thread a nut on the opposite end of the axle
(freewheel or non-controller side) until the outer side of the nut is flush with the end of the axle. I then tap the end of the axle/nut with a large wooden mallet
(or use a large hammer with a block of wood placed against the axle/nut to cushion the blow and prevent damage) until the cover on the controller side comes away.
Once the cover is unstuck you will need to remove the nut to allow the cover, controller, axle and stator assembly to be removed as a single unit, as I presume those are the parts you will be replacing in order to convert it from MPIII to MP4.
Check out
this post for a description of how I did it.
However, if you just want to remove the cover alone, check out
this post for further information.
Alan