We had a flurry of emails on Friday claiming that Quake 4 featured an Australian R18+ rating on Steam, sparking speculation that Steam had started adding adult ratings to certain games on the service. After a quick call to the Classification Board on Friday afternoon we discovered what had happened.
In short - it was a mistake. After a quick back and forth with the folks over at the ACB we confirmed what we thought from the beginning - someone over on the Steam website had goofed.
After we notified the Classification Board of the mistake on Friday, someone at the team got in contact with Valve regarding the issue and, after quickly checking this morning, it appears as though the game has gone back to its original rating of MA15+.
Thanks to everyone who sent this in, and thanks to Ashley for the screencap.