Can't seem to add extra sheets to 4 page schematic design
I don't know how to do this using GUI, but you can run command EDIT .sX where X is a sheet number higher than last existing sheet. It will create a new one for you.

It won't help if you reached the sheet limit of your edition though. Try creating a new schematics and right-click there again. If "New" is not grayed out anymore that would be the case.