Board Thread:Dark Souls II/@comment-68.7.163.188-20150519095912/@comment-4511886-20150519123427

The increase is the same. So if +2 to +3 is a 5 damage increase in scaling, then +3 to +4 is, usually, still a 5 point damage increase in scaling. As such, the scaling between +2 and +4 would increase by 10.

The letter is there to give a very general idea of how well the weapon scales.