I'll throw my 2 cents in. You still need the 4 sq ft for the coop or sheltered area of the tractor and 10 sq feet for the run part at least if they are standard size chickens. However if your chicks free range outside the tractor they can have smaller run space.
The problem with the tractor is it can only be so large or it is too big to move. I have seen them that claim about 16 hens but do the math. One way to do with more space is make the run separate from the sheltered portion that way they can be moved separately and then put together. They both can have wheels. I have seen large light weight PVC pens on the web you can make.
Good luck,
Jeff