Well of course, one of the premium manufacturers is Trojan and the new Odysseys would be my choice if outfitting a boat for cruising today given their superior charge/discharge qualities over everything else. They do NOT make an 8D...they make a PC2250 which can be parralleled for 8D size/performance but is lighter for handling. They also make a rack mount with 8D type spec...the PC1800 which might be of interest to you.
May I ask...why the 4D? Is that being considered for space reasons or are you planning to use it for starting? 4D's are TYPICALLY not great deep cycle batteries as they tend to be made for dual use purposes/construction.
Also...why are you going for AGM? I'm a fan but ONLY in certain circumstances like full time cruising or space/orientation needs etc.
They tend to get fried by long term dockside float charging.
What is your daily use and how will they be charged?
The Odysseys can be found at Odyssey Batteries
Nigel Calder did a great write up on their unique construction and performance in Professional Boat Builder in spring 2008.
I owned a bank of east penn 4x8D's and was well satisfied with their performance in full time cruising.
I am a bit skeptical of the lifelines as there are too many people that seem to have had poor results...some people swear by them though.
I have seen nothing but positive reviews on the Northstars...though they do not make an 8D.