Hmm Warhammer 3rd edition was released in 1998.
In 1999 they released an Imperial Guard Codex (the one in which Schaffer's Last Chancers were all individual characters with special rules).
The newest Imperial Guard Codex (the one with the castrated and flavorless Last Chancers and the sweet new doctrine system) was released in 2003, which does predate the release of Warhammer 4th Edition which was in 2004. Technically I guess it is the second of the Third Edition Imperial Guard Codices...
and you're right, the latest IG cover has the same markings as the other 3rd edition Codices. However, released not that long before 4th Edition came out, I wonder if we will see a real 4th Edition IG Codex or if they will just rely on a Faq to tidy it up a bit.
Perhaps like Samurai suggested with the Chaos book, it was designed with 4th in mind. I'm not sure, but I think the 2002 release of Chaos Space Marines was also the second Codex for the Third Edition.