College charging me for classes I couldn't attend?

You didn't pay for 4 hours. You paid for all your classes for the whole semester, because you reserved places in those classes and held your reservations long enough that they couldn't sell those seats to anyone else.

If you went to a restaurant and ordered a meal and ate a couple of mouthfuls of rice, you'd still have to pay for the whole meal, because they would have to pay to obtain the food and prepare it and serve it and clear it and so on. If you rented an apartment and lost the key after a month but you didn't tell the landlord and just slept in your car for the rest of the lease, you'd still have to pay all the rent. If you owned a car and you sold it to some guy and he crashed it 3 blocks away from your house, he'd still have to pay for the whole thing, not just for 3 blocks' worth of use, because you would no longer be able to sell that car to anyone else. It's the same thing.

When you're a little kid, if something is not your fault you may be protected from the consequences. When you're a grownup, sometimes bad things happen and they're not your fault but they're not anybody else's fault either, and you just have to live with what happened. And college is for grownups: they expect that if you aren't able to function as a grownup you will have someone assist you with things like doing the paperwork. If you had dropped the classes during the Add/Drop period you would have had a refund. If you hang onto your seat in the class past Add/Drop, you're on the hook. And being on pain pills is not an excuse. Plenty of people are on pain pills and other serious medication for much of their lives and they manage to act like responsible adults. People complete their classes successfully while being full-time employees and single parents, while undergoing chemo and having surgeries scheduled several times during the semester, while on really heavy doses of antipsychotic medications, while living in hospitals throughout the semester, while coping with the sudden deaths of both parents, and all sorts of other things. And in my experience people with serious medical problems who genuinely can't complete semesters but who take care of their responsibilities promptly sometimes find that colleges are willing to work with them, but people who think that a medical problem absolves them of any responsibility to act like adults generally do not. You are presenting yourself as a member of the latter group.