That is quite obviously untrue. If the Yankees were to play your local Little League team, would it go four games or seven? The greater the difference in quality between the two teams, the likelier it is it WON’T go seven games. A seven game series can be decided by a single play; a sweep, by definition, can’t. Luck can play a big part in a 7-game series; in a 4-game series luck clearly wasn’t as big a factor.
I would also point out that if you examine series that were clearly upsets, they’re just as likely to have gone seven games as four.
To answer the OP - if they were going to fix the World Series, aside from the fact that it could not possibly be kept a secret because you’d need both teams in on it, if they did do that sort of thing we would expect almost all World Series to go to six or seven games. But that is not the case:
2002 - 7 games (Angels 4, Giants 3)
2001 - 7 games (Diamondbacks 4, Yankees 3)
2000 - 5 games (Yankees 4, Mets 1)
1999 - 4 games (Yankees 4, Braves 0)
1998 - 4 games (Yankees 4, Padres 0)
1997 - 7 games (Marlins 4, Indians 3)
1996 - 6 games (Yankees 4, Braves 2)
1995 - 6 games (Braves 4, Indians 2)
1993 - 6 games (Blue Jays 4, Phillies 2)
1992 - 6 games (Blue Jays 4, Braves 2)
1991 - 7 games (Twins 4, Braves 3)
1990 - 4 games (Reds 4, Athletics 0)
1989 - 4 games (Athletics 4, Giants 0)
1988 - 5 games (Dodgers 4, Athletics 1)
1987 - 7 games (Twins 4, Cardinals 3)
1986 - 7 games (Mets 4, Red Sox 3)
1985 - 7 games (Royals 4, Cardinals 3)
1984 - 5 games (Tigers 4, Padres 1)
1983 - 5 games (Orioles 4, Phillies 1)
1982 - 7 games (Cardinals 4, Brewers 3)
Of the last 20 World Series, 8 went the distance and 4 were sweeps; without doing the math it seems to me that 4 sweeps is actually more than you would expect by sheer random chance, since randomly a sweep should only happen once every eight years. The spread of results seems pretty normal to me.