- Is it possible if I could get 4 letters of recommendation?
- How far in advance should I notify those who are writing my letters?
- Does activity on sites like Quora, Stack Exchange, and Reddit's AskScience (as well as a personal webpage/blog of one's research) count as public outreach?
To riff of off JeffE's comment:
Although only three letters of recommendation are allowed to be considered, you should always have an "emergency" writer on backup just in case one of the other writers can't submit the letter on time for whatever reason (sudden illness, job change, etc.). You don't want to miss out on the opportunity because somebody else dropped the ball.
You should notify them as far in advance as is logistically possible. You should already know who your letter-writers will be; you should find out how much advance time they need now, so that you can plan accordingly in the fall.
While most of those sites would count as public reach, I don't think Academia.SE would fall under that rubric. Something discipline-specific, on the other hand, would.