IPv6 is required for all Matter devices, either actual or spoofed. It’s part of the specification. (Matter was originally called Project ChIP, Where the IP was for IP addresses.) that’s true whether they are connected by ethernet, Wi-Fi, or thread.
That’s detailed in the community FAQ:
FAQ: What do I need to add a Matter device to the SmartThings app? Do I need a bridge router device?
Technically you should be able to use Matter with Ethernet only, no WiFi, but that doesn’t mean zemiSmart and SmartThings are set up for that option. And it’s still using IPv6 addresses.
which model SmartThings hub do you have?