If the four men are evenly spaced around the edge of the object and if the center of gravity of the object is precisley in the middle (and hence equi-distant from all four men) then the 50 Kg/person calculation should be pretty accurate. But in the real world it often doesn't work out quite so ebevenly. Most objects are odd-shaped, so the center-of-gravity is not precisely in the middle. Plus it's possible that two men on opposite corners may do more lifting than the those on the other two corners. For example, it's quite possible for two men on opposite corners to lift 80 Kg each while the other two are lifting only 20 Kg each.