According to Grand Seiko:
The caliber number is a four-digit number that indicates the model of a movement (mechanical part of a watch). The Grand Seiko watch is mounted with an exclusive caliber. The mechanical caliber number starts with “9S”, the spring drive caliber number starts with “9R” and the quartz caliber number starts with “9F”.※Some quartz caliber number starts with “8J” or “4J”.
The four-digit model number on the case back is the caliber number.In the case of mechanical models, the attached “Grand Seiko Standard Inspection Certificate” is printed with “Caliber No.” So, the caliber number can also be checked on the certificate.
Source: Grand Seiko 9S caliber manual