I never had 4 20-game winners on one team until the past month, and now I've had two teams do it. The first doesn't really count since it was a prog team and I had a lot more salary than nearly every other team in the league. But the more recent team is kind of impressive, I think. It's from an 80M single-season twist theme (I've got the 2006 Dodgers), and at the 154-game mark we're 107-47.
Here are the 20-game winners (all have between 230-246 IP):
2002 Odalis Perez, 23-5, 2.32
1995 Greg Maddux, 22-8, 1.50
2002 Derek Lowe, 20-9, 2.57
1998 Brett Tomko, 20-12, 4.84
Tomko appears to have done it with smoke and mirrors -- the league ERA is 4.49, so he's really got no business being 20-12.