Fire Department spokeswoman Mindy Talmadge says concrete being poured on the roof of the six-story building struck the workers Wednesday morning, covering them in sludge. Two of the workers fell about 30 feet after being hit.
All four men were taken to San Francisco General Hospital, where spokeswoman Rachael Kagan says three are listed in serious condition and the fourth is in fair condition.
Kagan says the workers range in age from early 20s to late 30s. Their names haven't been released.
All four men work for Nibbi Brothers General Contractors of San Francisco. A message left with the company wasn't immediately returned.
State workplace safety investigators and city building inspectors are at the scene.