The max torque on our seat masts is 7nM and the Bontrager preset torque wrench is 5nM so he is not over-torquing anything.
The correct torque is the minimum needed to fix the seat mast securely without going over the MAX torque spec for the BOLT. So if 4Nm is sufficient to hold the seat mast, then why crank it to 5Nm. The OP stated that he always used the 5 Nm preset key, yet the seatmast still damaged the seat tube. Assuming he did everything correctly are you saying he should confidently crank it to 5 Nm again?