Fourth Doctor's regeneration

"It's the end. But the moment has been prepared for."

- The Fourth Doctor's last words.

The Fourth Doctor underwent his regeneration into the Fifth Doctor after falling from the Pharos Project telescope. (TV: Logopolis) This was the fourth regeneration of the 12-regeneration life cycle beginning with the First Doctor. (TV: The Time of the Doctor)

Anticipation
After receiving a summons from Gallifrey, and telling Sarah Jane Smith that he had to leave her behind to answer it, Sarah Jane asked the Doctor if the summons meant he would regenerate again, to which he voiced his own uncertainty. (TV: The Hand of Fear)

After crashing the TARDIS on the way to Shada, the Doctor briefly believed he had regenerated. (PROSE: Doctor Who and the Krikkitmen)

After seeing the Watcher in a dream, the Fourth Doctor thought of his next incarnation. (PROSE: Into the Silent Land)

Regeneration
As Doctor anticipated his death while dangling, his mind flashed memories of, a Dalek, the Nomad Cyber-Leader, the Captain, Davros, a Sontaran, a Zygon, and the Black Guardian. After the fall, he saw Sarah Jane Smith, Harry Sullivan, the Brigadier, Leela, K9 and Romana's first and second incarnations. While surrounded by his companions, the Doctor assured them that the moment of his death "had been prepared for", and beckoned the Watcher towards him. The Watcher merged with him, and the Doctor's form faded into the Watcher's with his fifth incarnation emerging from the Watcher. (TV: Logopolis)

Alternate realities
In the Warrior's universe, the Fourth Doctor's life ended prematurely when he, after destroying the Dalek incubators in the Genesis Incident, was mortally wounded by a prototype Dalek. The Doctor was enabled to regenerate by drinking an Elixir of Life provided by Narvin, becoming the Warrior. (AUDIO: Dust Devil)