I'm sure you'll love it. It's a great place. Also slightly cooler than other parts of Australia at that time of year
Be sure to see Wineglass bay. And if you are up for camping, I can highly recommend the campground right next to it. Absolutely fantastic location - wallabies come up to your tent, a beautiful little creek and one of the most beautiful beaches in the world right there. Only thing, I'm not sure how popular it is that time of year, since I didn't go in peak season. Might want to check if you would consider that.
In any case, if you don't want to camp, at least find some nice longer hikes to go on. There's some beautiful stuff to see. I think 4 nights will give you enough chance to see the a bit of the island.
And yeah, islands off of Tasmania are also fun. I went to Mariah island. I think you can even overnight in the jail there if you're up for that