Enfield State Park is just down the road from Ballarat, just go down Colac-Ballarat Road. I used to go out there a bit, it's alright, nothing super extreme, just beware of a couple of mud holes though. It is mainly used by trailbike riders, but all the tracks are accessible to 4be's. A few extra tracks will open up after track closures finish up (around end of October), I've added a link to an old map of mine, it's very rough though:
There is also state forest that backs onto it with more tracks, but I haven't spent much time playing in there.
Then you have Wombat State Forest, which is not too far from Ballarat either. This place is massive and you can easily spend a few days in there, keep your bearings because you can become lost quite easily and there is a heap of water down there at the moment and the bog holes can be pretty hectic. From memory you can access it via Dayleford and it starts off as a double diamond track, but cant for the life of me remember what road you access it from.
Also, there is the Otways which has a heap of tracks open, some of the terrain is very wet and slippery at the moment. I might be goin out to the Otways this weekend if your keen on coming along. You might get away with AT's if it isn't raining, but you will struggle on a few sections depending on the size of your tyres and if you have any suspension modifications.
Hope this has been helpful mate