Suffering from radiation poisoning, the Third Doctor, unable to regenerate on his own, used the TARDIS to artificially regenerate. This "Fourth Doctor", in his only adventure, sought battle with his enemies, the Daleks. (STAGE: Doctor Who and the Daleks in Seven Keys to Doomsday)
However, the regeneration was unstable, and after adventuring on Karn, he reverted back to his third incarnation. The events surrounding this "regeneration" were erased by tampering in the Doctor's timeline.