Its also my recollection (but a fuzzy brain), that the older style chargers (without the fancy multiple charging modes) don't really care about the state of charge and will try and pump the 4 A into the battery anayway.
I have both types, and the older style charger often gets things working again, when my newer style charger says "insufficient voltage." Sometimes, the heavy "jolt" from a larger charger is enough to get the battery charging again--so Dave's suggestion is very worthwhile.
For your info:
Many of the 6-volt deep cycle batteries used in motorhomes and "fivers" have amp-hour capacities of about 200AH.
So, using your 4A charger could easily take as long as 25 hours or longer to reach a full charge!
Good luck with your battery ...