I've got to confirm, is that a 4-bank charger or a single-bank 4-stage charger?
In any case, you need a separate charging bank per each battery bank. You do not need to separate the batteries that are tied in parallel within each individual battery bank. If they are identical batteries, of identical age, discharged together, and charged together, they are functioning as one large battery, and do not require isolation.
There is always some increased risk, but still a very small risk, with multiple batteries, that one bad battery can take down the entire bank, so you can manually disconnect and diagnose each paralleled battery in isolation, once in the Spring and once in the Fall.
If isolation where to be required, you could use solenoids or simple On/Off battery switches between batteries. Diode devices aren't really an option because you can lose close to a volt, and that last volt is critical on a Kinetik AGM.
Let's say as an example, you have three paralleled batteries on the house/stereo bank and one lone battery on the starting bank. With most contemporary marine 4-bank smart chargers, you would wire one bank to the lone starting battery and wire the other three charger banks to the three paralleled house batteries. The three charger battery banks would not function as three independent banks, instead they would combine and function as one large charging bank with a much larger capacity to service one large battery bank consisting of three batteries. This is normal.
