Shawn,
Another option might be to separate them; two in one location and two in another. This way, you can distribute the weight a bit but the downside is the extra heavy wire that you would need. I used "Werker" brand from Batteries Plus and they seem to be holding up OK. I have 4 six volt, 215 amp/hrs each. I believe that the Trojan brand is regarded as the best wet cell 6v golf cart, but I agree about the price and that's why I went with Werker at around $90 ea.
I built two battery boxes that went below the quarter berth, pretty much where the original batteries were and a little aft as well. The downside is the extra weight concentrated on the port side which enhances the listing to port a bit. In the future, I may add two more 6v aft of the engine as well, but I will shift the water heater aft a bit so I do not interfere with the access to the stuffing box. I still keep a starting battery but I may eliminate it to save weight plus I truthfully haven't used it in many years.
I like your idea about the batteries under the port setee for quick access, but this is a little off center to port as well and may not help with the port list that much, maybe a little. The area aft of the engine may be the best for balancing the weight. Here's a link to "Windchime" ...they relocated the batteries aft.
http://windchimetimes.us/Look under photos/boat projects