Why is the name IPv6 and not IPv5 as successor for IPv4?

On any IP header, the first 4 bits are reserved for protocol version. So theoretically a protocol number between 0 and 15 is possible:

4: is already used for IPv4

5: is reserved for the Stream Protocol (STP, RFC 1819 / Internet Stream Protocol Version 2) (which never really made it to the public)

The next free number was 6.

Hence IPv6 was born !!!!!!!!!!!!!