The demo is available via code redeem on the Playstation network. Basically anyone who pre-ordered a copy of FNR4 got a voucher card with a unique code on it, and when you enter that code the download for the demo starts. Those that didn't pre-order can play the demo 2 weeks from today.
The action is very fast paced, they really increased the frame rate compared to FNR3. The animation is very fluid but the style is less realistic than in FNR3, the fighters seem almost like caricatures of their true selves. Backing away is definitely harder than closing the distance. If you want to gain any kind of distance you need to pepper the opponent with jabs.