The maximum number of Chinese stamps that I have next to any of their visa stickers is 7, so I can't tell from experience, but I strongly presume they will just stamp on the next (available) page. Otherwise your multiple-entry visa would effectively be limited to four entries, which is something we would be aware of were it the case.
One possible pitfall is if you have no more (empty) visa pages available in your passport. You might well be told to get new pages / a new passport for your next entry. A friend of mine had to get new pages attached to their passport in a slightly different case (needed a whole page for a new visa sticker, but same logic applies).
Immigration officials in China have always taken great care to keep their visa stickers and stamps concentrated neatly in a set of pages for me, so I have no doubt that they have already worked out some system for what happens when you hit the 8th stamp. Actually I would even bet that your visa sticker might be attached next to a double page, that is still empty?