Welcome to the Forum!
Generally, a new thread will get more responses than tacking on to an older one, but both are completely "legal".
In answer to your question, the difference between camper A that is 4K dry, 6K GVWR and camper B that is 4K dry and 5K GVWR is simply the suspension / axles / tires used and the maximum weight the camper is designed to carry. We always suggest GVWR in calculations because nobody goes camping in an empty camper and GVWR is closer to what the camper you actually pull will weigh. If it weighs a bit less, it is a safety margin. It is amazing how quickly a camper can "gain weight."
Note also that the PUBLISHED max tow for an older truck (pre 2014) assumes a base model, NO options except max tow package, one 150# driver, NO passengers, NO cargo and a full tank of fuel. Every pound of options, accessories, passengers and cargo must be deducted from the max tow.
Note also this max tow really applies to a pile of bricks on a flat bed trailer, NOT a 10 ft tall, 8 ft wide travel trailer. The advertising department does all in their power to make this number as big as possible for advertising. It does NOT mean YOUR truck can safely pull that weight. It means they make one similar to yours that can.
Thanks for asking!