Consider the following scenario. You have an application that implements an api with asp.net webapi.
You are setting up a new Identity Server 4 to use with your infrastructure. You need your api to work with the Identity server.
If it was IdentityServer 3, you would use IdentityServer3.AccessTokenValidation.
If your api was on .net core you would use IdentityServer4.AccessTokenValidation
But given that your api uses old style asp.net webapi, not even owin, and your Identity Server is .net core based, do you have an option of making them work together?