I did some touring in France with a pretty full Carradice Nelson, with a Bagman mount, on the carbon rails of a Romin Evo and a fake Enve carbon seatpost, and I weigh 90kg. All items still alive and in one piece.
Don't actually do that though. Bikepacking bags are better.
The seatpost is supporting 70-80kg of human, I doubt 4kg of luggage is going to bother it