Heap (Friday Night Dinner, Spy) and Bhaskar (The Kumars at No 42) will join the Brockman family for a 40-minute special, revolving around the colourful family's Christmas party celebrations.
[Mark Heap in Friday Night Dinner - left / Sanjeev Bhaskar - right]
Things, as usual, don't run smoothly for the family as gran turns up unannounced, Jake has fallen in with the wrong crowd, Karen is busy chatting with online friends and Ben is hosting games of blind football and swingball with the guests.
The show, written by Andy Hamilton and Guy Jenkin, features Hugh Dennis, Claire Skinner, Tyger Drew-Honey, Ramona Marquez and Daniel Roche.
A fifth series of Outnumbered will follow the Christmas special in 2013.
The show's producers have denied comments from the cast and rumours in the press that the fifth outing will be the sitcom's last, claiming that "no decision" has been made on its future.