We have a number of options how you can experiment or get started with IdentityServer4.
We have a demo site at https://demo.identityserver.io that runs the latest version of IdentityServer4. We have also pre-configured a number of client types, e.g. hybrid and authorization code (with and without PKCE) as well as implicit and client credentials flow. You can use this site to try IdentityServer with your favourite OpenID Connect client library. There is also a test API that you can call with our access tokens.
Here’s a repo that contains all permutations of IdentityServer3 and 4, Katana and ASP.NET Core Web APIs and JWTs and reference tokens. We use this test harness to ensure cross version compatibility. Feel free to try it yourself.
Our CI feed can be found here.