There are numerous threads on this, ExpiredLoch. You may wish to regale yourself by reading, for example, this one: How to Handle Quitters.
Right in these moments, I'm in my first-ever 18-hole multiplayer 4-some on WGT.com. The first player quit on hole 5; the second player quit on hole 7. Just as I wanted to type, "The remaining 2 of us are moving ahead to hole 12", the only remaining fellow player quit, too. I'm at +2 over par with 7 holes left to go, but I'm not gonna play them; after all, this was supposed to be a multi-player game, and 7 holes are quite many to play on one's own.
While it has seemed next to impossible to successfully finish a 9-hole 4-some, I suppose finishing an 18-hole 4-some on WGT.com (with fellow players selected at random) is from the realm of sheer fantasy. It may well never happen, ever, to anyone. Accomplishing that might be a more difficult feat than winning the Diamond Award for most consecutive birdies (which is 18).
Let's hope WGT will think of something to deter quitters, indeed.