A mild amount of water soaked through an upstairs floor into the ceiling below it due to a window being open in the upstairs bedroom during a storm.
The ceiling of the room below is slightly damp along one edge with faint water marks showing up. Renters were in the house at the time so it has been 4 days since this happened AND it took the four days for the symptoms to show up on the ceiling. I am currently drying out the room with the ceiling damage with a portable electric heater plus dehumidifier so will be very dry and room temperature will be at about 90 degrees. Do you feel mold has set in and I need to do more than this before I use appropriate primer and repaint?

No, mold will not set up that fast. Plus, the heat will kill the mold. I would still spray some bleach on the surface before priming.