My offer letter included a $3,000 increase after 1 satisfactory evaluation. I worked 4 years before receiving an eval.

The short answer is "probably" but, of course, the "devil is in the details."  It's great that you have the promise of an increase in writing.  I would have expected that your evaluation would have occurred before 4 years elapsed.  If the company has a handbook that says it does annual evaluations or something that shows the standard is more frequent evaluations, then you might be able to argue that you should have had the raise earlier -- provided that your performance earlier was satisfactory too.  If you've left the job I would say you truly have nothing to lose and everything to gain by raising a claim, either via a letter to the employer or by filing suit with your local magistrate.  The claim is covered by the Pennsylvania Wage Payment and Collection Law.  Remedies for unpaid wages include the back pay, penalty of 25% or interest from the date the payment was due and attorneys fees.  Under certain circumstances the owner or other decision-makers of the company can be personnally liable.  Suing them personally, if that is supported by the facts, is often a powerful inducement to them to settle with you.