do you mean MS-MPEG4 v3? (i believe MS-MPEG4 gets called S-MPEG4 in GSpot)
First of all its not really MPEG4, but kinda like it. So your encode is not really MPEG4 compliant. (but that is of course only a worry if you care about such things

)
Its also kind of old, so i expect it to be worse than DivX5 or XviD, but ive never seen MS-MPEG4 in a codec test so i dont really know...