This option is -4 as the short option and --ipv4 as the long, added in curl 7.10.8.
IP version
So why would anyone ever need this option?
Remember that when you ask curl to do a transfer with a host, that host name will typically be resolved to a list of IP addresses and that list will contain both IPv4 and IPv6 addresses. When curl then connects to the host, it will iterate over that list and it will attempt to connect to both IPv6 and IPv4 addresses, even at the same time in the style we call happy eyeballs.
The first connect attempt to succeed will be the one curl sticks to and will perform the transfer over.
When IPv6 acts up
In rare occasions or even in some setups, you may find yourself in a situation where you get back problematic IPv6 addresses in the name resolve, or the server’s IPv6 behavior seems erratic, your local config simply makes IPv6 flaky or things like that. Reasons you may want to ask curl to stick to IPv4-only to avoid a headache.
Then --ipv4 is here to help you.
--ipv4
First, this option will make the name resolving only ask for IPv4 addresses so there will be no IPv6 addresses returned to curl to try to connect to.
Then, due to the disabled IPv6, there won’t be any happy eyeballs procedure when connecting since there are now only addresses from a single family in the list.
It could perhaps be worth to stress that if the host name you target then doesn’t have any IPv4 addresses associated with it, the operation will instead fail when this option is used.
Example
curl --ipv4 https://example.org/
Related options
The reversed request, ask for IPv6 only is done with the --ipv6 option.
There are also options for specifying other protocol versions, in particular for example HTTP with --http1.0, --http1.1, --http2 and --http3 or for TLS with --tslv1, --tlsv1.2, --tlsv1.3 and more.