[Al Skierkiewicz]

OK,
Now we're getting into some really heavy stuff, pun intended! There is a lot of marketing hype in what we are discussing here so let me add some to Keith's post and to answer some of the questions raised by Andy.
In the deep cycle batteries you are topping off with water, distilled water I hope, you are replacing the water that evaporates during the normal charge/discharge cycles all lead/acid batteries experience. The battery temp is raised during charge and discharge causing some water to be released in a gas form in the battery. During charging, as current passes through the electrolyte, some bubbles form, rise to the surface and carry small amounts of water vapor with them. In a normally vented battery this vapor leaves the battery. In a sealed or maintenance free or call it what you will battery, there is sufficient space above the electrolyte for the vapor to condense and return to the cell. In all sealed batteries there is some kind of vent arrangement to prevent the battery case from cracking open any time the internal pressure rises. If the pressure increases because the battery was left in the sun, or because the electrolyte has frozen, there is very little water vapor lost. If the pressure was caused by charging, some vapor will escape when the battery vents. Over time this lost vapor will reduce the weight of the battery.
There are vents in our batteries, they are just hidden. The gell additive just keeps the liguid in when you turn the battery on it's side. That is a big factor in using these batteries in our competition. You cant turn them over and spill the acid. It is not an overly strong solution of sulfuric acid but it can be bad in eyes and on clothes. (clothes will disolve where there was acid when they are washed.)
Finally, the 4 amp chargers are in the range specified by the battery maker as being the highest charge current. This rate takes into account the amount of internal pressure reguired to "pass gas", a sufficient margin to prevent internal damage due to metal distortion of the plates and a quick enough charge time to maek them attractive power sources for general use. These batteries see service in such applications as exit signs and safety (loss of power) lighting and ham radio operators find them useful for portable operation preferring to charge them with solar cells.
To make matters worse there is probably a failsafe vent that is designed to open wide but only once. This is for the user that may have a voltage regulator fail during a charge or for extreme conditions of discharge or temperature, and is simply there to prevent explosion. (shrapnel and acid, lead contamination all really bad stuff)
Hope all this helps, thanks Keith for the added info.
Good Luck All