Differences for between releases are described in the Upgrade Guides. The big changes to PF syntax occurred with 4.7.
Here are the applicable Upgrade Guides. You are not planning to upgrade, but you will still find them helpful for things like PF.
http://www.openbsd.org/faq/upgrade47.html
http://www.openbsd.org/faq/upgrade48.html
http://www.openbsd.org/faq/upgrade49.html