Those 4 bolts and perhaps some baked on grease are all that are holding it in. If the slam method isn't working prehaps the prying or twisting methods will. A little liquid wrench with a few minutes of soak in time couldn't hurt either. I don't like recommending heat.
You could also try reinstalling the wheel (without the brake drum) using washers as spacers if necessary. Then use tread surface of the spare tire to strike the sidewall area of the installed tire with a swinging motion.
That trick also works when changing tires if you encounter a rim that is still stubbornly stuck to the hub once the lug nuts are loosened or off.
The links below show what the part looks like when not installed.
Don
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