Random thought: Playing a game, one person says a word and the next person makes a new word by changing only a single letter: E.g. Rain — Raid — Paid — Pain — Gain — Grin — Grit — etc. Can all 4-letter words in the dictionary be connected like this? Do some miss out? How many?

Not exactly what you're referring to but this paper uses this idea to teach about fitness landscapes, and references a tool that I think could be used to measure some degree of what you're referring to journals.plos.org/ploscompbiol...