I'm no JFTR expert, but I'd have to think after four years and you being out of the reserves now your window of opportunity to resolve the matter has closed.
However, recommend you re-engage the Navy Personnel unit that services the last reserve center you were involved with and work with them.
Here is a link to DOD Comptroller guidelines--you may have to hunt and peck thru it, find a hotlink to a document or directive that is on point for your issue: http://comptroller.defense.gov/fmr/09/09_08.pdf
I am licensed in the Commonwealth of Virginia, answering your query does not create an attorney-client relationship and I AM NOT providing you legal advice. You should speak with an attorney to whom you have provided all the facts, before you take steps that may impact your legal rights--or rights to recovery of damages. I wish you the best of luck with your situation.
I would agree that contacting the pay or personnel office where you were last assigned would be a good place to start. You could contact JAG but they represent the government and would not have to assist you. The Air Force has a claims office that handles certain claims against the government. I'm sure the Marines/Navy has a similar office. You might try that route. But as the other responder indicated, its probably too late.
As for hiring a private attorney, you could certainly do that but the real task before you is finding the right person to fix the issue. You may have as much success at doing that as would an attorney. Attorney time can be expensive so you need to weigh that against the amount of money you think you are owed.
Providing this general response does not create an attorney client relationship.