Richard Beckinsale was in the musical "I Love My Wife" when series 4 was being filmed, so he couldn't do it. He died a year after series 4 aired. You may be interested in the film of "Rising Damp" that was released in 1980 after finishing these episodes.
Richard Beckinsale hadn't died when this was made, he just wasn't avaliable. They never actually said where Alan was supposed to have gone. Richard did die the following year, and tye Rising Damp movie was made with a new character filling the role. Its still a very funny show, but it misses Alan. I do wonder if he had not died if he might have come back to do series 5. But without him I don't think they had the heart to do it.
UK sitcoms just make you smile all the time. And out of all of them Rising Damp, Only Fools and Horses, ALLO ALLO and One Foot in the Grave are the best. They closely showed the world how we operated and saw life. Brilliant Sitcoms all of them. The Only USA sitcom that truly comes close to these Uk Sitcoms is = Married with Children..I love watching you, watching Rising Damp especially.
It was like Rigsby became a parody of Rigsby in the 4th series. Probably given too many lines to try and cover for the loss of "Alan" Not sure what happened to the writing but it really fell off.