Both releases contain an improvement to work around broken authoritative servers sending replies without the “authoritative answer” (AA) bit set.
The 4.5.5 release contains a fix to an issue where an insecure domain with signatures records could be marked as bogus due to a missed zone cut and a fix to the aggressive NSEC(3) cache handling of denials of DS records.
The 4.2.x release is EOL and the 4.3.x and 4.4.x releases are in critical fixes only mode. Consult the EOL policy for more details.
We would also like to announce that starting with the 4.5 release we will stop supporting systems using 32-bit time. This includes 32-bit Linux platforms like arm6, arm7, and i386.
We are grateful to the PowerDNS community for the reporting of bugs, issues, feature requests, and especially to the submitters of fixes and implementations of features.