How to pronounce“421”in English

I am currently anti-theft, and the most important keyword when it comes to how to fight is this diagram. For example,this comes from a paper called " The Computer 421st Century" written by a man named Mark Weiser in 1991, andthis is exactly the scene we are seeing now. Someone is here, and they are using something like a whiteboard to talk, and the people listening are some looking at it