Two 6vdc golf cart batteries are more or less the equivelent of one 8D. So four golf cart batteries is the equivelent of two 8Ds, one for house and one for start.
Since it was built our boat was set up with two 8Ds only and it worked okay. But we were always a bit concerned about having enough house power.
Several years ago we changed to six golf cart batteries, four for house and two for start. This has in effect doubled our house power and we've found it very nide to have that cushion.
We also have a generator and an inverter/charger, but we still are much happier with the extra house battery power available. While we are still frugal with power we don't have to be AS frugal now.