The Complete DA module series is now available as Print on Demand from DrivethruRPG. The final module, DA4 Duchy of Ten just became available in this format. It had previously only been available as a PDF.
DA4 notably is the only module in the series that was not credited to Dave Arneson, but only to Dave Ritchie who had been credited as co-author of the previous three modules. Arneson later said that, unlike with the previous three adventures, he had not been consulted for this one. On the other hand, much of the adventure background repeats and expands upon elements that can be found in earlier instalments of the series. When Arneson's company Zeitgeist Games detailed these lands, they stayed fairly close to the descriptions of the Duchy as shown in DA4.
Read my review of DA4 from a few years back here.
Get your copy of Duchy of Ten here (Non-affiliate link)