On my server, my IP address appears as IPv6. However at who.is, it appears as IPv4. Why?

I have a list of IPv4s that I'd prefer to ban from my site. I've noticed however something strange. My server sees my own IP as v6, whereas if I go to who.is or a similar service, it appears as IPv4.

Why is this? And is there a way to convert the v6, which I know maps to a v4 address, back to v4?

Update: Within a span of mere minutes, I've now observed my IP address being provided to my PHP code as both v6 and v4.