Which 1981 serial featured the Fourth Doctor's regeneration?

The Fourth Doctor’s regeneration was featured in the 1981 Doctor Who serial Logopolis.

Logopolis is Tom Baker’s final serial as the Fourth Doctor. During the closing events, the Doctor is badly injured after falling from a radio telescope while trying to stop the Master’s plan. He reaches the point of death and regenerates into the Fifth Doctor, played by Peter Davison.

Broadcast in four parts from 28 February to 21 March 1981, Logopolis is the final serial of Doctor Who’s 18th season. Its story involves the mathematical planet Logopolis, whose inhabitants create calculations that help maintain the structure of the universe. The Master disrupts that work, threatening the universe with collapse.

The Five Doctors is a later 1983 anniversary special and includes the Fifth Doctor, but it does not show the Fourth Doctor’s regeneration. Castrovalva follows Logopolis and is the Fifth Doctor’s first complete adventure. The Keeper of Traken comes immediately before Logopolis and establishes important events involving the Master, but the regeneration itself takes place in Logopolis.