My fender tag has a C4 option as well, and I was able to get the fender decoder to work correctly, but it registered as an unknown option. I have a hunch it means seat belt delete, since I found CO4 on a fender tag or build sheet for 69-71 is: Seat Belts - Delete All Standard. I also tried plugging in different numbers under the C, but none of them were known options.
Here is what you get when you plug in your codes to a fender tag reader:
CAR: Plymouth RoadRunner 2 Door Pillar Coupe
ENGINE: 383cid 4-bbl HP V8
TRANSMISSION: 4-Speed Manual Floor Shift
TIRES: F70x14 Red Side Wall, Steel Belted
BUILD DATE: January 02.
AXLE: 3.23 Rear Axle Ratio, Sure Grip
INTERIOR: High Trim Grade, Vinyl Split Bench Seats. Red Interior.
PAINT: Monotone Red Paint.
OTHER: Burgundy Metallic Upper Door Frame Color.
MOLDINGS:
25: Drip Rail Mouldings
47: 'B' Pillar Mouldings (for coupe)
ABC OPTIONS:
C4: Unknown Option
F0: Decor Package
R1: AM Radio
abc OPTIONS:
---
Hope this helps!
-Mr. Moe-par
1968 Road Runner, 1968 Coronet, 1968 Coronet, 1969 Dart