This is a very good and clear explanation of how bitcoin type blockchains work. It could easily have been made simpler and more general at the same time by omitting the proof of work aspect, which is not even discussed actually. The "begin with four zeros" rule is completely arbitrary, it could be any prefix of any length depending on the necessary amount of work that you want, including zero. And with an empty prefix we still have a blockchain with all the properties presented here that hold.