The NWN coup de grace is different from the PnP version, but there is a logical reason for this -- it is a game mechanic to allow low level characters to survive low level mobs. The sleep spell is the most powerful spell a low level character (4th level or lower) has access to, insofar as it is a big AOE spell that can affect creatures up to 5 HD. However, it only lasts 3 rounds + 1 round/level. As a consequence, if you were to use it on a mob of 5 or 6 low level creatures, you might not be able to hit normally and damage normally to kill off the mob if you had to do regular attacks. With coup de grace set up as an insta-kill but restricted to 4 HD or less, a character could survive a small mob of goblins (for example), but not power their way through an ogre. It really is just about survivability at really low levels, since high level characters would rarely use a sleep spell, nor would they have any trouble dealing with monsters of 4 HD or less.