4.8 and 4.9 had few user visible changes to PF (but the logging system was cleaned up), bug fixes, IPv6 related fixes
The Next Big Thing: New traffic shaping subsystem (aka the beginning of the end for ALTQ), at first prio keyword for per rule priority (5.0)
The proxies (ftp-proxy, tftp-proxy) changed to divert(4), use divert-to in rules (5.0)