The -4 option controls the primary connection done to the proxy/host, it does not limit the name resolve of the remove host.

Example:

$ curl --proxy socks5://proxy.example:8081 https://ip.me -4

Will make sure to connect to proxy.example using IPv4. While using -6 insists on IPv6 to the proxy.

You could very well do Ipv4 to the proxy and send and IPv6 address to it (like you ask for) or send an IPv4 address to the proxy when speaking IPv6 with it.

curl actually has no option for forcing the IP version of the address for the remote name to get passed to the socks proxy.

The -4 option controls the primary connection done to the proxy/host, it does not limit the name resolve of the remove host.

I didn't know that. That's tricky and not something I'd expect. Also, see the doc:

"This option tells curl to use IPv4 addresses only when resolving host names, and not for example try IPv6."

A key property of curl is to have different options for different actions and not chain them, so that you can mix and match at your own choice.

Thinking further, I think it is reasonable to apply the IP selection also on this resolve to do what is more likely asked for. A future improvement could be to introduce a more fine-grained selection.