Depends on what you need, you have to realize that 500 of the square feet needed is outside and all the processors can share that space. The space needed indoors depends on the processors needed, two processors would only require about 1000 based on the new design 1500 at the most. I suggest you have a quick visit to the JBI recycling facility. At the back of the building there is a very high space where several processors can be placed because having the processors reduces the space needed to process the plastic, and to be honest much of the space is not used. It's also simple to add what are basically a tin building (which is all the current factory is) for around $50,000. You also have to understand that much of the cost in providing the space is already budgeted in transportation and landfill costs.
You are also mistaken, the sites JBI uses already deal with plastic, they already have loading docks, the additional employees are those just dealing with the processor, at the very most we are talking about 6 at one time (including admin etc...), I think they can squeeze them in, the employees dealing with the plastic are already employed by the partner.
I agree that finding sites suited is no easy thing to do but for different reasons that are more related to new technology and JBI having to prove what they can do. Negotiations do take a long time as you have suggested, so it's a good thing JBI have been in negotiations for over a year with some of the their potential partners. A lot of the leg work is and has been done.
Where did you come up with the 4000 sq' feet number, it's great to say you read it somewhere but where. The 1000 sq' feet I used is from JBI, that would include 500 sq' feet outside and me adding footage because of the increased size of the processor, which is why I suggested 1000-1500 sq' feet indoors for two (which share some parts).