If I'm understanding your question, you want to replace your house bank with 4 new 100 AH AGM batteries? That would be fine if they meet your overall amp hour requirements. You would be losing about 40 AH. Another number to consider when selecting batteries would be there RC number or reserve capacity. The RC rating is more accurate when the battery is used for high amp-draw devices such as inverters and windlasses. The AH rating gives a clearer picture of how long a battery can
carry lighter loads like powering lights and electronics. This could be e reason for the $120 price difference.
The AH rating tells you the number of amps that a new 12-volt battery at 80 degrees (F) can deliver, multiplied by the number of hours, without falling below 10.5 volts.
The RC rating is the number of minutes that a new, fully- charged 12-volt battery at 80 degrees (F) can carry a 25-amp load and maintain a minimum voltage of 10.5 volts.
For instance, a 100AH battery can deliver five amps for 20 hours because 5 x 20 = 100. The AH figure for most marine deep-cycle batteries is calculated at a 20-hour discharge rate, and this is important because the amount of amps a battery can produce is not strictly a linear measurement.
The higher the amp draw, the fewer total amps a battery can deliver. A 100AH battery can deliver five amps for 20 hours, but it cannot deliver 25 amps for four hours even though 25 x 4 also equals 100.
Connecting two banks with different capacities in parallel is technically fine since the batteries will be operating at the same voltage. Charge and discharge current will be shared, based on capacity. It is best if the batteries are of the same type and age. Broadly speaking, you can parallel batteries without problems, and the charge controller will look after them. Just make sure you give them plenty of charge. If the system tends to operate at less than a full state of charge, adding new batteries to old will probably just result in the old ones pulling the new ones down and everything getting sulphated.
