Each 6 volt is about 200 AH. 2 together get you the 12 volts and still the 200 AH.
A 12 volt battery, around the same size, will give you about 100 AH. 2 will give you 200 AH. Thats 1/2 the 4, 6 volts.
Yeah, 2 batteries gives about half the power of 4. Either way you get about 200AH for each two batteries.
It's not the voltage that makes the difference, and you can't change the chemistry of a lead acid battery by re-packaging it. It takes a certain amount of lead in an acid solution to produce x amp-hours @12v, and its the same whether you put it in 6v boxes or 12v boxes.
A typical GC2 (aka golf cart) 6v deep cycle produces 230 AH @ 6v, so two of them in series are 230AH @ 12v. A typical Group 27 size 12v deep cycle produces about 115 AH, so two of them in parallel are also 230 AH. Both contain about 62 lbs of lead. The GC2 6v is fairly short but tall, while the G27 12v is a bit longer but not as tall. One shape may fit better than another in your battery box or bay, but the total volume of battery will be about the same for any given amount of AH.
What does make a difference is whether the battery is a deep cycle or not. Most of the 6V sold are "golf cart" deep cycles (motorcycle and tractor batteries excepted), but a lot of 12v are not, despite being labeled "marine/RV deep cycle". Forget marine types and get a quality deep cycle 12v from Trojan or Lifeline. Expect to pay top dollar, though.
6v deep cycles are often more competitively priced, so on a cost per AH basis, it 's had to beat the 6V GC2 deep cycle.
If you choose AGM technology batteries, the price differential between 12v and 6v levels out pretty close.