If you think your computer can handle it you can make a LAN multiplayer game and connect to your own game 3 times. You can launch multiple instances of BG3 from the bin folder which the default location should be c:\SteamLibrary\steamapps\common\Baldurs Gate 3\bin or whatever drive you installed on. I did this, but had to turn down my settings on BG3 to be able to get through character creation on all 4 instances at once.
The only notable thing is the character portraits are made based on your graphics settings so you might end up with slightly bad looking portraits but this will allow you to create a fully custom party.
