we assume each person works at the same rate x qt per hour

when all 4 are working in 2 hours they collect 6 qt

4*x*2 = 6

x = 6/8

x = 3/4 qt per hour

let t be the time needed if 3 of them are working to collect 6 qt

3*x*t = 6

3*(3/4)*t = 6

t = (6*4)/(3*3)

t = 8/3

t = 2 h 40 min

3 people will collect 6 qt in 2 h 40 min.