Can my employer force me to give medical information for a non work related injury that occurred 4+ years before employment?

Yes, if it relates to your being able to perform the job or there is a chance you could be re-injured in performing the job.

It depends on why they're asking the question. For example, if you've filed a workers' compensation claim, that is somehow related, they are likely to get the information, but in most cases, no.

Only if you need them to modify the job somehow to accommodate the injury. This is a disability accommodation question, in my view. They can inquire about your ability to do the job, and if you request accommodation, they can inquire about the nature of the disability. Depending on the situation it might be worth calling some lawyers or paying for a consultation to get more specific advice.