The fourth Dr Who adventure 'Shadows of the Vashta Nerada' is available from the BBC site (free to UK residents) HERE.
The previous adventures are still available (free within the UK). Each like the new one is about 330Mb download size and need about 1.5Gb - 2Gb each hard disk install space.
You can download these - 'City of the Daleks', 'Blood of the Cybermen' and 'Tardis' HERE
They all need really quite a fast PC, a good graphics card and plenty of RAM, and the latest Java version installed. Get this HERE.
If you live in the UK don't under any circumstances pay for them on any third party site.
Outside of the UK
Non-UK residents can legitimately buy the set of all four Dr Who adventures HERE for 10.95 Euros from Direct2Drive. (That's just over US $15 or less than US $4 per game!)
Direct2Drive are the only company licensed by BBC Worldwide to sell these adventures outside of the UK. Any other companies offering these for sale are acting fraudulently unless they link the sale to the Direct2Drive site.
[re: other posts - "There might be ways non-UK residents 'could' get working copies 'free'" - note - this would breach BBC copyright]