It is a new series of Family Guy, they just didn't tell anyone untill like last week. Same with American Dad, FGS4 was only 10 episodes and ADS1 was only 5 episodes. These new ones will be counted as a whole new season.

http://www.epguides.com/FamilyGuy/

Also:
FOX pulled one of the new season episodes as it contained "several" references to a hurricane - and placed it further down the season - replacing it with another episode out of the same season.

Perhaps this is the reason for the 'nuking' or numbering confusion. But yeah - even familyguy official site states only 10 episodes for season 4.