Unequal treatment of employees for non-job-related reasons is a violation of Federal labor law. Termination because of an injury four years ago is definitely "unequal treatment". Unfortunately, in order for the employee to do anything about it, it literally requires the making of a "Federal case" out of the matter.
There are government agencies available that might be able to help. You can file a formal complaint with your company, which would be a first step before contacting an outside agency. This is called “exhausting your administrative appeals”. The formal complaint would set forth the facts of what has happened, why it is inappropriate or not factually correct, and what you want done differently. Since you have already been fired, there seems to be little point in such a formal appeal.
Once the formal complaint is filed you are headed down a road which could involve confrontation and retaliation issues. If you get no satisfaction and thereafter file a complaint with the Equal Employment Opportunity Commission, it is likely you won’t need a lawyer, the implied threat of the EEOC sometimes results in resolution through mediation. This would at best result in a change of behavior of the boss, but wouldn't involve the possibility of monetary damages. But since you have already been fired, confrontation and retaliation are no longer issues, but the EEOC won't be able to reverse the firing.
If your company is a federal contractor another agency, Office of Federal Contract Compliance Programs (OFCCP) might be of assistance. But the same point applies, you have already been fired. That leaves you with one remaining avenue of recourse.
The average person is ill-equipped to fight this out with their employer. Your last resort, and sometimes most effective tactic, is to find a labor lawyer who is willing to take your case on a contingency basis. This means that no up-front payment is required, the lawyer will get paid from the lawsuit proceeds, if there are any.
The bigger the company is that has done this, the more likely that an attorney will take the case on a contingency basis.
Preparing for such a case is a lot of work, not just for the attorney but also for the person who was wrongfully abused, slandered, harassed, or terminated. Be sure that you want to go through with the added effort of a lawsuit before you start a lawsuit, the last resort to change the situation.
Good luck with this.