I don't think that there's a discount on the exams UNLESS you register as a CompTIA member (which I think is done on a per-company basis, though I assume you could do so with an independent "business"). The CompTIA members used to get discounts on the exam vouchers.
The current version of CompTIA Network+, exam code N10-005, was released Dec. 1, 2011. I'm not sure why the 04 version is still out there, but it goes away at August 31, 2012. Unless you think that you can be ready and pass it by then, I'd just aim for the 05. The only reason I can see for taking the 04 is if you had specifically studied and prepared for the 04 and don't know the material that has been added for the 05.
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CompTIA A+, Network+, Server+, Security+
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MCSE:Security 2003
MCITP:Server Administrator
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