The Fourth incarnation of the Time Lord was played by Tom Baker, for seven consecutive years, which is the longest living incarnation in the Doctor Who history.
The Fourth Doctor brought an eccentric style to the role, with his trademark scarf and a fondness for Jelly Babies, which captured the publics imagination. This incarnation is still regarded as the most recoginsable and most popular.
The Fourth Doctor begins his time by attempting to distance himself from UNIT and to a lesser extent, Earth. The Time Lords still send him on occasional missions as he travels with Sara Jane Smith. During this time he meets his new Arch Enemy, Davros.
The Doctor travels with Sara Jane until he recieves an urgent telepathic summons from Gallifrey, and must go there alone as humans are not allowed on the planet. The summons is a trap set by the Master, having used all his regenerations and is now little more than a withered husk.
The Master then frames the Doctor for the assassination of the President of the High Council of Time Lords. In order to avoid execution the Doctor invokes an obscure law and declares himself a candidate for the office, giving himself the time he needs to defeat the Master.
The Fourth Doctor is a wonderer, thrilled by adventure and discovery, unpredictible and always five steps ahead of everyone else with an off-beat humour and light hearted mood.
His time ends as he travels through an alternate Universe called E-Space, revealed to be a Charged Vacuum Emboitment (CVE) which has been created by the mathematicians of Logopolis as part of a system to allow the Universe to continue on past its point of heat death.
As the Fourth Doctor investigates this, he begins experiencing ominous feelings and spots a white-clad entity, The Watcher, observing him. After succeeding in stopping the Master from disrupting the CVEs and destroying the universe, the Fourth Doctor is mortally wounded when he falls from the Pharos Project radio telescope control tower.
As he falls he utters his last words: "It's the end -- but the moment has been prepared for." The Watcher is revealed as a manifestation of the Doctor's future incarnation. Before the eyes of the Doctor's companions, the Watcher merges with the Fourth Doctor, regenerating him into the Fifth Doctor.
The surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us.