This sounds to me more like a dying battery that is no longer able to hold a full charge. Rechargeable batteries do not last forever and after some time will loose capacity.
This can manifest in many ways and I am not surprised by this particular manifestation.
The red flashing light is probably trying to tell you that there is a fault with your battery.
The reason you are likely seeing it quickly go down to 80-90% is probably due to the battery voltage "relaxing" after it has stopped having power applied to it.
4 years is around the right sort of time frame to be looking at getting a replacement.