Two workers can finish building a play set in 18 hours. How long will it take 4 workers to build 3 play sets?

If two workers can finish building one play set in 18 hours, then one worker can finish building one play set in 2 * 18 = <<2*18=36>>36 hours.

Four workers can finish building three play sets in 36 / 4 = <<36/4=9>>9 hours. Answer: \boxed{9}.