You're right! The buckets must be more than 1/4 the mass of the table. In this case they must be 1/4sin(angle between rope and table) the mass of the table. So if the angle formed when the buckets are in contact with the table was 60 degrees then the weight of the buckets would have to be about...
If the angle between the rope and the table is theta when the buckets are in contact with the table then the weight of each bucket (m) must be at least equal to the weight of the table (M) divided by 4sin(theta).
m = M/[4sin(theta)]
See attached PDF.