Even without any additional features I think $4/month is very reasonable for any game, certainly one that is well done, continues to improve and offers support for problems. That's a McDonald's combo meal per month. Skip it and live longer.
You know, I was just about to write that I think it's too expensive but then I read this post. I agree. About $4.50 AUD isn't a bad deal for a game that I really enjoy. I do wonder if they had other payment options, if they would get more supporters.
Personally, I think more needs to be done with the aesthetics side of the game. Once hooked, the game is pretty awesome, but you won't get many of those people who come for a quick browse.