There are a lot of people raving about the the new Optima batteries. They are expensive but so far I've not heard anything negative about them. As for chargers, on board is the way to go. When you get home, you plug it in and forget about it until its time to go fishing again. The good ones are self regulating and once the batteries are charged they go into a maintenance mode. The 2 most importantant things you can do to prolong battery life is keep the water level up and recharge them immediately (within 24 hours) after each use. There has been a lot of discussions on the subject here, you might try doing search. BTW, 4 years use out of a marine batttery is outstanding!
Fair Winds and Following Seas
Bill H. PTC USN Ret
Chesapeake, Va