Am assuming you will be using a carpet pole to handle your carpet rolls. Carpet rolls most often come in 12' to 15' lengths. Which means the effective load center of that 1500 lbs load is at least 72" to 90". plus the lost load center (thickness of the mounting plate & any operating clearance) of the rug ram/carpet pole. If your max. load weight is 1500 lbs. You will need a forklift that has a basic rating of at least a 4500 to 5600 lbs. @ a 24" load center (you need to confirm with your dealer). A 4000 lb. basic rated forklift seems to be a bit light. The only lift truck company that offer a 3 wheel electric above 4000 lb is Clark they have a model (TMX 25) that has a basic rating of 5000 lb. @ 24" L.C.
You have to remember that the effective load center of the load will not be exactly half of the carpet roll length - it will be longer as the roll won't be butted up against the face of the carpet pole to protect the end of of carpet roll from being soiled. You also need to provide the the dealer with the physical characteristics (weight, lost load center, etc) of the rug/carpet pole your will be using.
Yes driving up a 16% grade ramp with or without load will consume battery power faster. Typically most ramps are set a 10% grade (where possible).
PS: There is more to battery maintenance than just maintaining proper electrolyte level.