A "fully-charged" battery in my Canon XTi goes almost immediately to 1/2 and then is good for about 40 shots. One battery is a Canon and about 4 years old. The brand of the other is unknown, but ia year newer. Both act the same. Battery or camera
Why don’t you get in touch with the Canon a/s service center for proper clarification?
May be four years is enough for you to get a new battery patterning to the duration of the battery.