As of march 2009 AqBanking 4.x (and Gwenhywfar 3.x) is the most recent library family for online banking by Martin Preuss. It is not source-compatible to AqBanking 2.x. Hence, older Gnucash releases did compile and run with AqBanking 2.x. You can get the most recent AqBanking release from the AqBanking homepage.
This page did describe the necessary changes needed for Gnucash supporting the most recent AqBanking versions. Since the release of Gnucash 2.2.7 Gnucash fully supports the newest AqBanking releases. Hence this page is empty now. If you are interested in the gory details of what has been done already, please checkout this page's history.