The pros try to dry hop just before end of fermentation to reduce oxygenation while yeast is still active, as introducing dry hops will also introduce oxygen. At least that is my understanding. It is also my understanding that for me at least, on a homebrew level, that fermentation 5gallons is usually complete in 4-5 days, but I am always too busy during work week to check the daily gravity readings. This practice has lead me to check at the end of a week and say wow! It's done! But I secondary/condition for a few more weeks, with the notion that yeast is still doing other things to the wort/beer but not primary fermentation any longer. This would lead me to believe if a pro was homebrewing that they would add at four days and try to be sure that they purge oxygen out of hops as much as possible (which I don't think is really possible at all unless you racked into a purged keg..).
But, what I do is rack during secondary conditioning or during kegging, or both. I will try to do it at four days one of these times. I suppose that you would need to ensure that at four days, your primary fermentation wasn't going along at a furious pace, or you might not be at tail end of fermentation after all, so I would take my advice with that grain of salt if you will.
Time is also variable. Some say they get astringent favors after more than 5 days, but I've left the hop sack in for a month or longer without ill effect, when I've added to the keg. I believe the Pliny beer gets dry hopped twice, and once for like 10-14 days then again for 5-7 I think.
TD