I've been a couple of times a few years ago, to get close to the circuit you will be shooting through the safety fence, so you will end up high in the stands to shoot over it. I wasn't shooting myself as it was before I got my camera, but I do remember there being a gap in the fence for the official camera guys use. There was a guy there will a long lens.
Walk down to the front rows and follow the fence, I think it was on the corners of either side of the start/finish straight, so they can shoot down the straight. My memory isn't so clear on if "anybody" could use the gap, or if it was somehow separated from the public.
Also, assuming you have a paddock pass, you walk through a tunnel under the circuit to get to it (when it is open to allow access). When there you have free range to walk around the cars and watch the crews prepare for racing in the garages. Don't forget to photograph the pit girls that hold the signs
I went to see the ASCAR racing which I thought was fast, until I went again to watch Indy Car, they were VERY quick!
I reckon that I would sit high up in the stands opposite the start of entry to the pit lane, so that I could shoot cars as they slow to leave the circuit and enter the pits. From there you might also get a good view of the fourth bank as they pretty much approach you around it, assuming a long lens.
Sorry my memory is fading of the circuit and stands!