"A 4D is a drop-in and cheaper."
Is the 4D a start or deep cycle?
Many of the ones I have seen were starts , that suck as deep cycle batts.
Don't be fooled by "marine combo" batts , there not great at anything.
True deep cycles have loads of amperage left even when down 50% , if there are enough batts combined that there is plenty of plate surface , they work fine when ganged for engine starting.
Try a start with the house batts while they are down , if the engine spins fast , your home free.
A second check would be a digital Volt meter on the starter motor , over 9.5V while starting and all is well.
I have often thought ALL deep cycles with a new starting aid batt might be a good solution for summer cruises.