You can mail a withholding notice to his current employer. Get a copy of the form from the clerks office and have it issued, properly. You won't get everything you're owed this way, trust me, but it's a start.
A Notice to Withhold Income (together with the Uniform Order of Support issued 4 years ago) sent to his current employer is a start that may result in payments (depending on the employer). However, you are probably best off getting an attorney to file a Petition asking the Court to hold him in "contempt of court" for failing to abide by the court order, when he had the ability to do so. If the court finds him in contempt, he may be ordered to pay your "reasonable" attorney's fees. In addition, if you have the current employer's information, you can likely get the court to order that support payments be paid directly through the State Disbursement Unit by garnishing his wages. The Court can garnish wages for current support and likely an additional amount toward the back support that has accumulated over the past four years. Lastly, you are likely entitled to interest for the back support that has not been paid.