I respectfully disagree. Increasing the air temperature 4° increases duct temperature via return air therefore decreasing the chances of reaching dew point.
The space between the first floor ceiling and second floor's floor. Is probably at 78°F and 55% RH, or higher. If the return air temp is 74, and the temp difference across the coil is 20 degrees. making the supply duct 54°F. It will still sweat, as the dew point or the conditions I posted is 60.5°F. Even if the temp difference between supply and return ws only 14 degrees, the supply duct would be 60, so just under dew point.
When posting in certain forums, knowing your location will help others give better feedback/advice/solutions to your questions.