I've never tapped a keg after 4 days, but if you use the method where you shake the keg, I suppose you could get it to carbonate that way. When I keg part of a batch and bottle part o it, I bottle from the keg. I use a Blichmann beer gun, but there are other options.
You can drink a legged beer sooner than a naturally carbonated one because you're not doing the second fermentation to produce the CO2. So, if your beer was fully fermented, a week or so on CO2 should make it drinkable. I still usually give mine more time, but that's largely because of the amount I have on hand.
Ok I messed up and forgot to add the brown sugar to the Voodoo recipe. Rookie mistake, in a rush to get brewing! Anyway, is it possible to open the lid and add it now? I'm 5 days into fermentation. Thanks in advance for the help. If I can't, I'm still hoping it's a decent brew.