My public IP is IPv6, but I thought I had a static IPv4

I have a commercial, small business service from COMCAST which includes I think 4 or 5 static IP addresses. I have one computer that is wired directly into the COMCAST modem and statically configured for one of the IPv4 addresses. If I do commands like "ip addr" it correctly shows the IPv4 address.

However, if I go to a public reflector like whatismyip.com, it shows my public address as being IPv6 and shows an IPv6 address.

What could be going on or how can I diagnose this further?

Yay, it's about time Comcast started deploying IPv6 to business customers. Well, several years late, but I digress...

IPv6 is the default Internet protocol and has priority over IPv4. For most Internet services you try to access, if they are available via both protocols, IPv6 will be tried first. (And web browsers will relatively quickly fall back to IPv4 if a web site isn't responding via IPv6, but that's another discussion.)

If you want to see both your global IPv4 and IPv6 addresses, you'll need to use a site which was designed to be dual stack aware. A few years ago I built one myself for just this reason, which you can find at https://myip.addr.space/ . This will show both your global IPv4 and IPv6 addresses, along with identifying which protocol your browser automatically selected.

Your machine is probably has both. It is called dual stack implementation. This way your computer is reachable from IPv4 and IPv6 at the same time.

To ensure that you have IPv4 connectivity, you could use a computer outside of your internal network to ping the IPv4 address. (Assuming that there is no firewall to block the ICMP message).

If you want to just do a quick check, without actually leaving your network, you can use a service like this one to try to ping your machine. If you have a DNS setup for IPv4 and IPv6 then its enough to ping that, but you can also just ping individual IP addresses.