There have been 8 playoffs under the current format of 4 hole aggregate score. The first was in 1989, won by Mark Calcavecchia. Here are the results, and whether the winner finished first or last (or some other position, since more than 2 can be in the playoff):
1989 - Mark Calcavecchia over Wayne Grady and Greg Norman. Calc finished in the middle of the three; Norman finished earliest, Grady was in the final group.
1995 - John Daly over Constantino Rocca. I believe Daly finished one group ahead of Rocca.
1998 - Mark O’Meara over Brian Watts. I can’t recall if they were in the final pairing together (I think they were), or if O’Meara was a group ahead (I can’t recall, nor does the BO site say whether O’Meara, Parnevik or Furyk was paired with Watts).
1999 - Paul Lawrie over Justin Leonard and Jean Van de Velde. Lawrie finished first; Leonard finished a group ahead of Van de Velde, who was paired with Craig Parry the final day.
2002 - Ernie Els over Thomas Levet, Stuart Appleby and Steve Elkington (remember him?). Els played in the final group of the day, finishing even par for the day, as did his playing partner, Soren Hansen, the only two among the top 13 who did NOT break par that day. Appleby, Level, and Elkington finished more than a couple groups ahead; Appleby was first to finish, but Levet was the one who remained tied with Els after the four holes were done, requring a sudden-death repeat of the 18th hole.
2004 - Todd Hamilton over Ernie Els. Both played in the final group.
2007 - Padraig Harrington over Sergio Garcia. Harrington was two (?) groups ahead of Garcia (remember Garcia watching Harrington put it in the burn on 18 while he himself hacked around on 17?).
2009 - Stuart Cink over Tom Watson. Cink was three groups ahead of Watson.
So the results are kind of all over the place. Nevertheless, I maintain that anytime there is a same-day playoff (4 hole or sudden death), the person who has played more poorly is more likely to lose. Since a person finishing earlier will have had to shoot better to be in the playoff, ipso facto they are more likely to win the playoff.
Perhaps someone could look at the stats on sudden-death playoffs for the PGA and US Open and see if that holds true?