4M4 is the designation for an SH model. They didn't stamp all 17 characters in the neck until 1982, but began giving the bikes 17 VIN numbers on the plate. However, the DOT manufacturer tag or VIN plate should have all 17 on it. Example would be JYA4M4XXXBX004507. The JY means a Japanese Yamaha, and the "A" is the motor division, so it's a motorcycle. 4M4 means SH model. The 10th character is the model year. "A" being 1980, so "B" is 1981, so on and so forth. 7 and 8 are model specif items. Like it has a 2 cyclinder engine. The 9th digit is a "check" number. The 11th is the assembly plant, then the line sequence number are the last 5. So yours is the 4,507th made.
Although, you should have a plate with all 17 on the neck.