It does sound like your bank, or a portion of it, has reached its end of life.
To confirm the fact that the bank has changed, and convince the owner to install new batteries:
1. What does the Xantrex battery monitor
report for charge efficiency factor (CEF)?
2. Did you take specific gravity readings while you equalized and if so, what were the temperature compensated readings?
3. Are there any cells that are using more water
than the other cells?
4. Do you have a digital voltmeter on board?
5. Check simple things like conductor integrity from the charging
source to the bank; switch position; conductor termination integrity; connection of the termination to the battery; etc. to ensure that you are, in fact charging
the entire bank.
Four years is about the average lifetime for a flooded lead acid golf cart battery, especially if it has remained on float charge for most of its life.
Hope this helps.