If you don't mind, could you please post a link to the $4 IR camera?
There are a large number of similar cameras on the market that ship from China/Hong Kong on eBay. YMMV, but if you've got a few bucks and are willing to wait, it may be worthwhile (depending on your use).
[0] http://www.ebay.com/itm/USB-50M-6-LED-Night-Vision-Webcam-Ca...
The cats in question are pretty different (one black, one orange), so discrimination in visible light is easy. Not sure about IR yet.