Following are speculations for the specials and season 5:

414. The Next Doctor
203. 251208 by Russell T Davies
Lots of spoilers now on The Next Doctor page.

If you're looking for the 2008 season 4 episode guide, please visit:

While travelling in a bus- the 8:12 from Aldgate - the Doctor and passengers are suddenly transported to another world. The Doctor discovers that the world is all that is left of a great race that had reached the pinnacle of achievement. Unfortunately the science the original inhabitants of the world had found resulted in their destruction and a fortean flicker that was now pulling people from space and time.

The Doctor does not have his TARDIS so has no way of returning. He has to find a way to disable the machinery causing the problem, keep the band of bus passengers together, and get everyone home, oh and protect everyone from a band of  psycopathic bugs called the Tritovores who want to use the power as a weapon

Lady Christina de Souza (an international jewel thief!) becomes his companion for this story. Played by Michelle Ryan.

UNIT makes an appearance as does Captain Erisa Magambo, last seen in Turn Left.

NB: Gareth Roberts has said he has taken some ideas from his book The Highest Science. 

Some rumours and news for the final 3 10th Doctor specials below.

416 The Waters of Mars
205  211109 by Russell T Davies and Phil Ford.

Ood, Mars, what more could you want? Oh. You want Ice Warriors too? Highly likely!

417 Last Days Suggested title
418 Nostalgia's End Suggested title
206  251209 by Russell T Davies
        010110

It's nostalgia-fest time and, even though the final episode of season 4 brought everyone back for what seemed to be the absolute last time, the final 10th Doctor episode is likely to do that again. Confirmed sightings of the Master, Wilf, Donna, Sylvia with confirmed attendances of Rose, Jack, Mickey, et al. Rumoured to feature Brigadier Gordon Alistair Lethbridge Stewart, Sarah Jane Smith, a Dalek, a Cyberman, an Ice Warrior, a Slitheen, a host of other aliens and a trial in a massive vault of aliens from Doctor Who, ala Star Wars. The Shadow Proclamation and the Judoon are also likely to be back along with a number of other characters from episodes the 10th Doctor has been in. Yes, the erstaz Doctor is supposed to be there too, along with Rose's baby!

Other rumours: Will we finally see Rose Tyler, Earth Defender, with her half human partner The Doctor?

This article says that several Hollywood names have been approached but have turned down the offer to play a major villain linking the 2 telemovies. Hugh Grant was recently approached to star as the villain. Julie Gardner confirmed that the first special will be screened during the Easter long weekend. >

Season 5 by Steven Moffat. Starring Matt Smith as the 11th Doctor.

Predictions for season 5 are below. Well, of course these aren't true! But if writers are true-to-form, this could be what we'll get!

501. About Time
207 030410 By Steven Moffat
A darker, edgier season with a much older (1000 years old?) 11th Doctor suffering from his loneliness. Two new assistants (male and female) join him and together they investigate time bleeding and other strange goings on in parks around London. The companions start to develop a relationship with one another.

502. The Dust of Stars
208 100410 by Neil Gaiman
Kylie Minogue is back as Astrid Peth! Somehow she's found a way to merge with an obsolete TARDIS and, being symbiotically linked with it, she's able to trace the Doctor. It's taken her about 100 of his years to do it, apparently. She needs to warn him about an impending disaster from the centre of the galaxy.

503. Scherzo
209 170410 by Rob Shearman
Time continues to splinter throughout the universe and the Doctor and companions fall through a crack into another universe with different rules.

504. Family Reunion
210 240410 by Stephen Greenhorn
Doctor-lite episode. The Doctor's daughter discovers a world shifted out of phase with the rest of the universe. She finds that her symbiotic nucleii allows her access and discovers Time Lords frozen in time. She finds a way of tracing active Tardii but it leads her to discover other problems.

506. Fractured Time
507. Merlin
212   150510 by Steven Moffat
         220510
People from the Doctor's past return to haunt him. The unstable black hole is causing ruptures in space time, allowing universes to bleed through from alternative realities. There is a real risk of the Doctor being sucked into one permanently.

508. Ice Time
213  290510 by Ben Aaronovitch
The Doctor becomes embroiled in a plot in King Arthur's court in an alternate reality. Unfortunately he is betrayed and trapped in a field of frozen time. Using parallel world jumping technology developed by Torchwood, it's up to the Brigadier and UNIT to rescue him, with the help of a few friends.

509. The Fans of Death
214  050610 By Steven Moffat
People around the world are being found dead in locked rooms, in places no one would ever be able to get to. The only thing suspicious in the room is an electric fan. Unfortunately, after being trapped in an alternate reality for hundreds of years, the Doctor is in no condition to help. He must rely on his friends and UNIT to battle a new and deadly creature that manipulates metal. Freema Agyeman to guest star.

50X The End of Time
2010 Christmas Special
219 251210 By Russell T Davies
At the end of the 2009 Christmas special Matt Smith's 11th Doctor catches a cold and it causes a temporary regeneration. He turns into... a woman!

It's still rumoured that Jennifer Saunders appears as a temporary Doctor either in the Christmas Special or the Children in Need special. Remember what happened in the first Christmas special when the Doctor lost his hand? Well, other things can happen in that period!

Other notes:
Steven Moffat will be the producer for this season and has said that, firstly, Billie Piper will definitely NOT be in any of the seasons. Secondly, he has not discounted River Song from appearing again and that she knows more than one Doctor. Thirdly, he doesn't plan to bring back any old monsters unless there is a really good story, and that goes for cameos, too. And fourthly, season 5 will be much darker.