There is no widely accepted definition or standard for the classification of different generations of blockchains and cryptocurrencies, and some sources may refer to a 4th generation of blockchains. However, this classification is highly subjective and not yet widely recognized in the industry.
That being said, some people view the 4th generation of blockchains as those that focus on addressing privacy, security, and sustainability issues, as well as further improving scalability, interoperability, and usability. Some of the technologies that are often considered 4th generation blockchains include privacy-focused blockchains like Monero and ZCash, and eco-friendly blockchains like Algorand and Chia that use proof-of-stake consensus algorithms instead of energy-intensive proof-of-work algorithms.
It’s important to note that these classifications are not set in stone and may change as the technology continues to evolve. The future of blockchains and cryptocurrencies is still very much in flux, and it’s likely that new generations of technology will emerge as the field develops.