At least buy "deep cycle" labeled 12V batteries or better yet two 6V batteries... Why do you want back breaking 4D batteries?

The Deka 904D (135Ah) costs significantly more than two GC2 6V batteries (215Ah's - 230Ah's and they are actual deep cycle) and more than two group 27DC 12V's (180Ah's and slightly deeper cycling than the 4D).

The 4D has less Ah capacity and is a lesser battery in terms of cycling even when compared to the 12V DC Group 27 or 31. If you don't have a Cummins 6B or Cat 3206 or larger engine there really is zero advantage to 4D or 8D batteries. In reality they don't cost you less they cost you more.... Boat builders slap them in, not because they are the right battery, but because;

A) They are cheap
B) The size looks impressive to Buffy & Skippy at the boat show...

At our local Sam's the 4D is $157.42 for 135 Ah's

4D - $157.42 / 135Ah = $1.17 per Ah (approx 250 lab rated cycles)

At our local Sam's the 6V GC2 is $84.52 X 2 = $169.04 for 215 Ah's

GC2 - $169.04 / 215Ah = $0.79 per Ah (700 - 1000 lab rated cycles)

At our local Sam's the 12V Group 27DC is $79.88 X 2 = $159.76 for 180 Ah's (350 lab rated cycles)

Group 27DC - $159.76 / 180Ah = $0.79 per Ah (350 lab rated cycles)