Without a separate trans cooler, I woudn't risk beyond 3500, that's 70% of your capacity.
Transmission fluid, filter, and install an oil cooler (bypass the radiator lines). Change oil, check your plugs and wires. Brakes must be in top shape.
Your trailer will have a net weight (empty), a gross weight (maximum INCLUDING the trailer) and load ratings of your tires (you should pack spares for it,which will add weight.
Unless your truck is wired for trailer brakes, you won't have them. Mine is, but I don't know if that's stock.
GO SLOW Do not do 70MPH. Do not try to keep pace with traffic. Right/center lane, go the speed limit at most.
I trailered in a 1994 Silverado from Carson City NV to Richmond VA. Max speed was 60 the entire way with a 7,000lb trailer. Center lane all the way (Right lane was dangerous inside cities due to the constant need to yield to mergers. You'll find opinions on this, but center lane travel was the safest for me.