I bought a couple of extra cameras like these ones http://www.ebay.com.au/itm/36LED-1-3...item41583edd15 from these guys and comparing the quality of these to some other ones I have had for a few years that cost me $$$ from Altronics they are great value for money.
If your looking for 4 cameras checkout http://www.ebay.com.au/itm/4-x-1-3-S...item41583ec989