A4559 is defined as: Coupling gel or paste, for use with ultrasound device, per oz.
The lay person description provides more information of what this code represents: Conductive paste or gel may be used on the skin at the site where electrodes are placed to improve conduction of electrical impulses. Coupling gels and pastes are used with ultrasonic probes and devices. The gel and/or paste acts as an agent to keep the probe from sticking to the patient's skin, probe cover, or other covers. The probes temperature can range from cool to hot and may require specific, high-heat gels or pastes. Ultrasound conductive coupling gel may be covered and separately payable if an ultrasonic osteogenesis stimulator is covered.
Obviously, this is code does not represent a patch, let alone a patch containing any medications, so this would not be the correct code to use for a diclofenac patch.