I’m confused…Are you getting 4v (or any voltage drop) across the red LED?
Are you getting continuity from the solder joint of pin 6 on the microcontroller socket to the LED?
Did you re-seat the microcontroller in it’s socket?
Did you test the polarity of the LED, as it’s installed?
While you are answering these questions please also check (and even re-flow) the solder joints on the microcontroller…specifically looking for tiny bridges around pin 6.
-Brach