On the website 'Den of Geek' lots of interesting facts about what the Series 4 Boxset commentary reveals have been posted online:
ON THE CHRISTMAS SPECIAL...
• The phrase "cache of info-stamps" is said (not revealed by whom).
• There's a character called Aubrey Fairchild, (the name of the Prime Minister in early drafts of The Stolen Earth) who meets a "terrible death".
• The Hub set, from spin-off series Torchwood, has been 'trashed' and used for filming in the special.
ON THE FUTURE...
• David Tennant refers to Steven Moffat as his "new boss", and their discussion indicates that they are already working together.
• The Moff prefers two-parters but this will not affect how he approaches the series.
• When asked if the title sequence for series 5 will change, the incoming show-runner jokes, "Should his face be in it?"
• The written (but not filmed) 'Shadow Proclamation' scene in The Stolen Earth, featuring monsters from throughout the series, has been requested by David to appear in one of the specials.
ON THE SARAH JANE ADVENTURES
• The production team couldn't find an actor short enough for Kaagh in The Last Sontaran so they had to make a new costume - thereby upping the cost of the episode.
• The actor who played the milkman from Journey's End is to appear in an upcoming episode.
• Catherine Tate wants a part (but not as Donna Noble), as her daughter loves the show.
• David Tennant wants to be the voice of 'Mr. Smith'.
and the biggie:
• David Tennant was to appear in the opening episode (in a sub-plot which would explain why K9 was not to appear in the show) but, at the time of recording the commentary (May 2008), this idea was binned. Julie Gardener states "it could still happen".
• On the Paul McGann TV Movie, RTD says, ".. so not what I would do, but I love it."
• Although RTD loves the Ice Warriors, he says they are "slightly generic".
• When asked if River Song is The Doctor's wife, Steven Moffat says "it's more complicated than that"
• RTD reveals that Harriet Jones died in The Stolen Earth despite Phil Collinson's protestations that she lived.
Also, below is a picture of a deleted scene from Journey's End, during which the Doctor was going to give Rose a piece of the TARDIS so that she could grow her own:
Again thanks to Planet Gallifrey and Den of Geek for literally everything in this post!