I bought a very sorry looking F4 last year and this was a problem as well. The problem in mine, which I suspect is common, is corrosion of the shaft that connects the little lever to the internal mechanism. Deciding I had nothing to lose, I first put a little lighter fluid around the point where the lever goes into the body, then took some small pliers, and very carefully started tweaking the lever back and forth, taking care not to do it hard enough to break anything. As the shutter began to move a little, I continued to wiggle it just a tiny bit more each time, until it came free. The secret here is never to push too hard, and to be patient. If it takes 50 or a hundred cycles, it's still way way quicker and easier than repairing it if it shears. The first bit of movement may be so small it is hard to detect.
This works, by the way, on a lot of things that are seized. It can take a long time, but as soon as you have any movement at all, you usually are winning the battle. All you need then is patience.
