"It is probably the least used 4 year old 12V marine battery on the planet."
And therein, my friend, lies your problem!
Batteries don't just die. They are murdered, usually by folks who don't know much about them. I don't mean to be critical of you, here. Most people don't know much about batteries.
A lead-acid battery (including flooded, gelled, and AGM types) will deteriorate if it is left in an undercharged state. Damage begins when the battery leaves the factory. Lead-sulfate crystals begin building up on the plates and, unless the battery receives a full charge, will continue to accumulate and will embed themselves in the plates, thereby reducing the battery's ability to accept and maintain full capacity. New batteries -- off the shelf -- can have a very considerable variation in their capacities due to this factor alone.
While there are other factors which contribute to battery deterioration over time (contamination, corrosion, PbSO4 buildup under the plates, running dry, over discharging or charging, etc., etc.), lead-sulfate buildup is by far the #1 culprit. And, it is the result of undercharging or leaving batteries in a less-than-fully-charged state.
As was correctly stated above, flooded batteries have a high self-discharge rate...as much as 3-5% per month. You must keep them fully charged to maintain their capacity. And, occasionally, it is useful to "equalize" them (i.e., to hit them with a charge voltage exceeding 15.5V for a 12V battery) for a few hours and let them gas vigorously. This helps to equalize the charge in each cell and to knock easily removable lead-sulfate crystals off the plates.
If your battery is four years old and has been treated as you say, I'd toss it. Get a new one and treat it right: keep it fully charged. Yes, keep the charger on when at dockside (assuming it's a marine multi-stage charger and that your electrical wiring is up to the job). Check the electrolyte level in each cell frequently, judiciously adding only distilled water when needed to keep the level about 3/8" above the plates.
If you treat your battery well, you should get much more than 2 years out of it. Five years is about right for a flooded battery which has been well cared for. Premium flooded batteries (e.g., the Rolls/Surette line
) will give you about twice that much service -- or more -- but they are costly.
Hope this condensed primer helps a bit.