I don't think you can use 800k/4m to say government is running at 80%. Some departments are affected more than others. Furloughs can have ripple effect. NASA, HUD, ED are the most affected according to data.
But clearly most of the government continues to run because either it was declared "essential" (a rather haphazard process) or is "self-supported" by fees, like the Post Office* or Amtrak.*
More detail about "essential" workers/activities: http://www.baltimoresun.com/news/maryland/bs-md-shutdown-ess...
*-ironically both lose money