It is entirely possible, and quite likely, that 4 year old flooded G-24 batteries are at end of life, especially if they have actually been deep cycled.
In the flooded form factors of G-24, 27, 29/30/31 and most
4D and 8D batteries are really more of a dual purpose battery yielding less real world cycle life than a battery specifically designed for deep-cycling purposes, even if the label says "deep cycle"...
What is a Deep Cycle Battery?
Also please don't be lulled into the understanding that a specific gravity test can tell you much about the state of health
of a flooded battery. All an SG reading can tell you is the batteries state of charge
or a cells SOC.
You can also derive battery SOC from a resting open circuit voltage reading. SG is best used to discern cell imbalances
and to help determine equalization schedules..
Even a battery suffering a 60% loss in Ah capacity, eg: 100Ah battery delivering just 40Ah's, can still put up a good SG reading. It's entirely possible, I see and measure it regularly, for that 100Ah rated battery to deliver a perfect SG reading when it can only deliver 40% of its original Ah capacity.