It sounds like the battery is toast, but do test it as Lokiyawl suggests. Four years between charges will kill most batteries, as they self-discharge much faster than that. Charging
every few months is a better plan.
Anode problems can come from improper wiring
, a non-isolated shore-power ground, neighboring boats with these issues, or a "hot" marina. If you can disconnect shore-power and keep your batteries charged with a solar
panel that might greatly improve your anode life. If you must use shore-power, install a ground isolator.
If those suggestions don't work, you have improper wiring, bonding, dissimilar metals in the water
, or something similarly difficult to deal with. Get expert help. Quickly disappearing anodes can lead to major problems, including dissolving thru-hulls, props, etc.