hi.
as i understand it, youre asking if you can stack up 4 melamine cages ? well. imo it wouldnt be a problem, but ofc it depends on on the length of the cages, contra the amount/weight of substrate your going to use.. you should be able to fix this by support "floor/roof" of the cage the a stick in the middle of the cage..
melamine is easy to work with, and wouldnt need a scetch that much. i would say you should drill before scewing, to avoid the melamine crush'. i just builded one my self, but right now i cant find out how to upload fotos, ill try to figure it out.. you are more than welcome to ask for any questions, and ill try to answer as good as i can. ( english isnt my mother language)..