Steve, Thanks for a very good question.
4 abandons in a standard game is very high for our site.
Many games (mike cold give the detailed numbers) go without a single Abandon.
Players who abandon a game get a big ding against their player rating, and several abandons will make it likely that a GM will not accept them into their game.
Players are limited to one game until they have completed their first game, but exceptions are made for those who prove reliable.
As always, we are at the mercy of real life.
Most Abandons are replaces, not CD'ed. Many players will find their own replacements if RL prevents them from continuing. Also we have the Light Brigade, a group of players who get notified when a game needs a replacement. This keeps our games moving with only slight delays most of the time.
Jason