You can gain access to Supernova features, if you already have basic GC4, buy buying Supernova at great discount. Very similar to how it would have worked with an addon
I am not sure how others feel, but I felt GC4 was an unfinished game that had support pulled from it before it was 'done'.
In a nutshell it feels like GC4 was abandoned.
You should not need to pay to continue getting support for a game that is not 'done'.
There are a lot of reviews out there on GC4 that cover the issues, so I'm not going to rehash.
I also think Supernova was released too soon, it is not balanced yet, and even the game scores and ladders seem wildly undone. The issues are all on the forum threads here. Things reported months ago still unresolved.
I get it, only so much time and man hours.