Historically, access to media devices was exclusive for performance reasons. Some systems have lifted that restriction now, others haven't.
For those that haven't, whatever asks for something first, gets it, and everything else has to wait until the first one releases it. So if the first instance of OBS grabs all 4, the others are out of luck until the first one either closes or deletes *all* of its references to them.
If your system is still exclusive, and you really need the same thing in multiple instances, you might use one of several plugins that are designed to send stuff between instances of OBS. They're not meant to work with anything else, but they do work for that.