F4MP VS GDLauncher

That mods dead it seems as stated on their github. But there is a new WIP mod called FalloutTogether. If you want more details about check out this YT vid. However, it's still early In its stages and is pretty unstable only giving you like 5 minutes of playtime before crashing.

If you are talking about F4MP (a lot of Youtubers were talking about it a year ago) it is no longer being worked on.

There was a multiplayer mod for FO4 in development a while back, but last I heard its been canned by the devs. Here's a link to the github page if you want to check it out, though it is set to read only. However, there's recently another multiplayer mod in the works called FalloutTogether. Here's a link to a youtube video detailing it and its installation.

There's also numerous mods for Bethesda games to add some kind of multiplayer experience. F4MP has been a thing for a while now.

I used to use GDLauncher, but I would not recommend installing a launcher that lets issues like this one go by for almost a year without being fixed: https://github.com/gorilla-devs/GDLauncher/issues/1288

Skip all that manual install crap - https://prismlauncher.org/wiki/ or https://support.curseforge.com/en/support/solutions/articles/9000196904-creating-a-custom-profile or https://github.com/gorilla-devs/GDLauncher/wiki

Another way to make it easy, a bit less easy than the prior but a bit more customization - https://github.com/gorilla-devs/GDLauncher/wiki

The latest GDLauncher beta added an option to allow users to manually download problematic mods. You can get it here (click the Assets dropdown) or wait a few days for it to be released as a stable version.

I would have recommended GDLauncher until this happened to me last week: https://github.com/gorilla-devs/GDLauncher/issues/1288

Maybe give a different launcher a try, I use GDLauncher to run modpacks

maybe you could try GDlauncher it allows you to

Personally i use GdLauncher since it's really simple to use, it also automatically installs open jdk for you, something that basically makes the game run better than normal java, and can still manually download mods that have third-party downloads disabled.

Which other launchers could I use instead? – The shunted devs have established PlaceholderMC as a temporary official replacement. There's also ATLauncher, GDLauncher, and PolyMC itself was a fork of MultiMC.