> The IPv4 32-bit address space should have been included in the IPv6 address space, instead of having 2 separate address spaces.

The entire IPv4 address space is included in the IPv6 address space, in fact it's included multiple times depending on what you want to do with it. There's one copy for representing IPv4 addresses in a dual-stack implementation, another copy for NAT64, a different copy for a different tunneling mechanism, etc.