On the upside, the salesman wasn’t trying to overload your truck.
Based on your numbers, you have a tow package, likely 3.92 gears (a plus if you do anyway).
4000lb max is ridiculously low for your truck, but comfortably, I’d want to stick to about 7klb max ready to camp, so 5500-6000lb max dry weight trailers.
Your intended use is a factor though.
High mile road trips, mountains or altitude, frequent towing, I’d want to be on the low end of that for ease of towing and comfort. Say 6klbs max.
Short trips, infrequent use, low altitude in the flat lands, I wouldn’t object to towing a camper up to the rated capacity of the truck.
Here’s where it gets more subjective. Some folks think you need a factor of safety of 100% and aren’t comfortable towing anything they can, figuratively speaking, “feel” behind them.
Others are fine with loadin er down and mashing the skinny pedal as far as it needs to go as long as they can keep it rubber side down.
And most are somewhere in between. Experience and what you “feel” plays into it a lot
And how hard you are willing to or will be working the truck.
