There is no way(*) to convert an IPv6 address to an IPv4 address as those two systems are used independently. (* except for very special cases which only rarely apply, see below.)
Why exactly did you want to convert an IPv6 address to an IPv4 address? A rough analogy for better understanding might be landline vs. mobile phones. Some people might have both and you can use both of their assigned numbers (IP addresses) to reach them, some people only got a mobile phone and no landline (or even the other way round). You can’t “convert” a landline number to a mobile number and vice versa, just like with IPv4 and IPv6 addresses.
Those “6to4 notation” and “IPv4-mapped notation” are regarding special use cases from the early times of IPv6. 6to4 notation was/is used to tunnel IPv6 over IPv4 links, and IPv4-mapped notation is used for internal representation purposes to represent IPv4-only nodes to IPv6 nodes. Nothing that would be of any help to you.
For your purposes, the longer IPv6 addresses are just like the older, short IPv4 addresses. They serve the same purpose, they are just longer because of the MUCH larger address range. If you want to read up about the why and what, there is a Wikipedia article for you: https://en.wikipedia.org/wiki/IPv4_address_exhaustion and another about the “strange” IP addresses you encountered: https://en.wikipedia.org/wiki/IPv6