This paper, based on the well-known problems and the most technical challenges in Blockchain space, studies ground-breaking and critical inventions of various blockchain protocols to give a taxonomy for the evolution of four public Blockchain generations. The first and second generations are well-defined by Bitcoin and Ethereum, respectively. The latest state-of-the-art blockchain protocols have been shaping the third and fourth generations, by their own outstanding innovations and distinguished architectural designs to solve limited capacity and scalability of Bitcoin and Ethereum. This work helps readers quickly capture historical evolution and innovations of public blockchains, envisioning the next advancements of Web3 as well as the Internet of Value (Internet 2.0).