I learnt to play 3 chords with the help of Bert Weedon's Play in a Day book way back in the sixties and , like you, still looking for the fourth chord.
I have a signed copy of that.
My parents were on a cruise and got to know David Lodge of Cockleshell Heroes fame (also Ice Cold in Alex).
He was a friend of Mr W. and following a conversation about how I had taught myself to play from the book, David talked to Bert and Bert sent a signed anniversary copy of the document to my folks sometime afterwards.