The diameter of the bars should be measured and compared with the stem. More than likely, the fit between the two was wrong. No carbon bar should fail because it was torqued to 5Nm or 6Nm instead of 4Nm.

After reading similar stories about stems and carbon steering tubes, I did some experiments of my own. I've taken cutoffs from steering tubes, clamped a husky stem (with two M6 clamping bolts) onto them and applied a ridiculously high torque with a long-handled hex wrench. I was never able to cause any damage. Most cases of damage result from a poor fit and/or raised metal along the edges of the clamp.