I understand what you are saying about the wire-vs-wireless requirements. We might be able to use wires but it isn't preferable. Just how much of a system would be required to support 4 wireless cameras, or maybe only 3?
Unfortunately, the best answer I can give you is... It depends
It depends on a number of things, specifically the camera, you can buy better cameras that do a lot of caching and compression and send less 'data' over the air and do some of the work for you. But you pay for that.
You can probably do 2-3 and you might be able to do (4).
If you Isolate this from your regular computer network so this wireless traffic and the resulting data doesn't interfere.
Now if this is a very sensitive area though, and not just thugs you're worried about, the 2.4 ghz spectrum that these things run in isn't hard to mess with and interfere with either which is another reason to avoid it altogether. Some that 'know's 'what they are doing will know what those cameras are and can easily defeat them.
That being said, signage, and dummy cameras are almost as affective as the real thing as far as a deterrent goes.
So without getting too technical...
You would need:
(4) Camera's
[These could be IP based cameras or not]
~~~ Wireless ~~~~
[A wireless Reciever]
a Good wireless Reciever (If it's standard 802.11) for IP based network cameras, you could try a Linksys, or off the shelf cable modem type router/wireless router.
Or a Wireless reciever capable of handling (4) Video Feeds, if these are proprietary non IP based cameras.
Then all of that goes into a view/DVR recorder like shown on that site.
The biggest difference is, if you do it with an IP based solution, the viewer/IP solution will probalby simply be Software for a PC.
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Another site to look at which has really cheap and sometimes Good stuff, is X10.com (Warning, it's pop-up heaven) but it's really home based cheapy stuff, but they do have some good stuff.
I hope this was helpful. It can still get complicated and can be a pain in the but to get good results. I would see if there is a decent Dial One Security Contractor in your area and contact them for a bid.
Dial One in Cincinnati is very reputable and won't stear you wrong. Hopefully in your area they are the same.
-MD