To the OP... The real question is if YOU are willing to take the risk and liability. I don't know how the laws work everywhere, but around me, the driver is the one responsible for their load and vehicle,... all of it. This also means you are responsible for understanding the weights and limits of your vehicle and adhering to them. I ruffled some feathers in another thread heading down a similar road, but say you are in an accident. Towing beyond the vehicle's rating would make you civilly liable, and no one will care what the dealer said.
This is a hot topic here in Ohio, because local RV dealers are pretty notorious for selling RVs that people don't have enough vehicle to pull. In some cases, the dealer's themselves have been pulled onto the carpet but always gotten away from any responsibility due to it ultimately falling on the driver.
I get that the dealer is saying they can make the rating the 4500# but unless they are going to write you up something official, that negates what is in the manual and on the door sticker, I would be concerned. Even this isn't a for sure "get-out-of-jail". IMO, this is all a civic concern and not a legal one, but it's still a concern.
Now, for the ability.... In spite of the fact I have another thread on my economic concerns while towing, I can tell you that our KL pulled our trailer (~1600 pounds) like it wasn't even back there at low speeds. In all honesty, it really impressed me.
My only concern (that has been addressed) is how deep you need to go into the water. Around here, the concrete gets REALLY slick bast the waterline. For this reason, I personally would consider leaving the system in "auto" as this puts the power bias more towards the front where you have traction.
As for the weight... If you can handle it to/from the ramp, you will handle it at the ramp. Backing down the ramp is more about the brakes and as soon as the boat touches water the trailer gets lighter. Same for pulling out of the water, the trailer isn't back to full weight until the boat is all the way back on the trailer and by this time traction should be plentiful.