IPv4 is the fourth Internet Protocol, succeeding early experimental Internet Protocols (some of which also integrated TCP functionality). IPv4 is a packet-switched communications protocol: data is divided into fixed-length (~1500-byte) packets which can be sent individually along a variety of communications media, shepherded by "dumb" routers which only need to know how to forward each packet to its next hop. Endpoints are identified by their IP address, which each router reads to determine the appropriate next hop. Unfortunately for the world, it was designed by academics as a still-fairly-experimental successor to the earlier experimental protocols, but became much more popular than anticipated and ran out of their control despite major limitations, most notably the 32-bit address space. There are only about 4 billion IP addresses, and now many more than 4 billion computers. IPv6 was designed to fix this and other things, but was standardized 15 years afterward, by which time it was already far too late. IPv5 is forbidden knowledge and must not be spoken of.
It's already far too late.