There are multiple ways to get it.
First of all, you either need 3 other people or 3 more PCs (maybe even VMs) that you can connect to the same network.
The easiest way to do it, if you can't get everyone to come to you with their laptops (PCs are harder to use because you need all of the peripherals as well), is to use Hamachi and connect to a local network over the internet.
Another possible solution (haven't tested it) would be to connect to the same VPN, which is easier if you host it yourself.
If you don't have the needed friends, but have a ton of money, you can buy the game 4 times and just join yourself - I'm not sure if this works if you're playing through steam family sharing in offline mode, I'll happily update the guide if someone can confirm that it works.
If someone needs the help with setting up Hamachi, feel free to message me, even though everything is pretty straight forward