If your GTR4000 still has the serial number that it was tagged with when it left our factory,  you should be able to find it on a silver sticker. Normally these are placed on the left panel of the unit, but you may find it on the back panel, right panel or top or bottom cover.

If you are still unable to find this sticker or tag that is clearly identified with "Eventide Inc", then at some point in this unit's history the serial number sticker was removed.