This. There is a reason 2/3s~ of the human race don't get cancer. Hint, it isn't because they were lucky.
That should be simple to test, by checking if four cups of coffee only lowers oral cancer or other types of cancer too. If it only lowers oral cancer but mysteriously no others, then this lowering effect has nothing to do with your "physical constitution".
Cancer rates are not entirely genetic, that much is already extremely well-established ... there are other major risk factors like smoking and obesity. Given the obesity link, and the fact that coffee also appears to lower your risk of diabetes, I suspect the effect may be coming from elsewhere in the coffee.