I need to connect an existing IdentityServer4 a FoxIDs tenant. How are IdentityServer4 configured as a FoxIDs up-party?
You are correct a IdentityServer4 client is configured as a FoxIDs up-party using OpenID Connect.
You can see how IdentityServer4 is configured as an up-party in the documentation. Furthermore, the documentation also reference sample applications showing how to do the integration.