SOCKS is a protocol used for proxies and curl supports it. curl supports both SOCKS version 4 as well as version 5, and both versions come in two flavors.
You can select the specific SOCKS version to use by using the correct scheme part for the given proxy host with -x, or you can specify it with a separate option instead of -x.
SOCKS4 is for the version 4 and SOCKS4a is for the version 4 without resolving the host name locally: