Well,
I read a lot of intelligent information on this post and some not intelligent information.
I think that what your problem is - is that instead of using your noggin, you want to buy something that will do your thinking for you.

If you have 4 batteries, and you connect it to a 10 amp charger, you will still place a 10 amp charge upon the batteries, but to the batteries, it will look more like a 2.5 amp charger - since it is dividing the current between them.

I have seen some cheap people in my life, but I will guess the OP takes the cake. If you think that a 10 amp charger is going to make your electric bill go up or down by regulating the power being applied to the battery at day vs night - you are only fooling yourself.

Until you understand a charge vs. a surface charge, you will never understand what events are taking place within the batteries.
I liked the analogy of someone using diodes to isolate each battery.
But a even better solution is to use those batteries as a battery bank, say on a transistor radio - police scanner would be a good example.

If you charge the batteries nearly full charge and you use those batteries for a couple of days or even a week at a time, you will create a cycle, which will keep the batteries at optimum power - slow down the calcification on the battery plates and keep the one good battery from being killed by the one junk battery that doesn't store as much power.

The other thing is - if you are stupid enough to use a trolling motor with a deep cycle battery until you run it down to nothing, then you ought to be willing to replace those batteries every 4 years, because even though they say deep cycle - you are squeezing the life out of them when you discharge them beyond 10.5 volts DC.

Your battery charger is not a power generating station - it doesn't work like 3 mile island. It will create a charge sufficient enough to recharge a battery to the point of where it is useable again and then it will reduce the amount of current, so it does not over heat the plates in the battery.
The fold back on the charger is sufficient enough to protect the batteries, but once the VOM - and this is the missing link in your hardware - once the VOM shows 13.7 volts - the battery holds as much power as it can possibly hold... Like a cup of water, what happens when you over fill it? The excess product spills out.

Batteries by nature are dirty.
The dirt, acid etc on the battery creates a discharge because the current flows out of the battery through the dielectric to ground.
The other thing I didn't read is what you placed the batterys on?
Wood is prefered, but some rubber mats might not be bad either, as long as they are thick enough and kept clean.
A heavy truck mud flap would probably work well.

The missing link in this post is to advise you not to connect the batteries either in series or in parrellel, but to PURCHASE a BATTERY TENDER...
This takes all the guess work out of keeping your batteries charged.

Even if you watch a couple of episodes of American Chopper, eventually you will see someone go to the battery rack and pull out a fresh battery from the rack. How do you think they keep those batteries charged?
Do you think that they use a mindless battery charger? Or do you think that they use a Battery Tender or some other commercial battery charger?

Do the math - if they build a dozen motorcycles a week, how many batteries does that add up to at the end of the year?