In other words: Do I have to update both my Macbook (from where I manage the remote OS X Server) and the Mac Mini (where the Server is actually serving) to Yosemite and OS X Server 4 at the same time?
What do you mean: "Nothing can go wrong: all your Keychains; SSH keys; Certificates; Open Directory settings; web apps; .... will migrate just faultlessly."?
I think my strategy will be to upgrade the Server machine first and only manage it locally until I am sure it's working perfectly. (Not an option for those without local access to their OS X Server.)