In the last few words, you get it right: "don't do anything unitl X Nm, then release." The FORCE is the point at which you want to release, not the velocity. What you're suggesting is that what we want to do is hold the "release velocity" constant, so we have to adjust the "release force" to correct for a change in mass. In fact, we want to hold the "release force" constant; there's no reason to adjust anything.
Your binding isn't supposed to release when you fall. You can break your leg standing up on your skis (beginners, in particular, do this more often than is entirely healthy); you can fall without any danger of getting hurt at all. Rather, you're binding is supposed to release when the force (stated in Nm or foot-pounds or whatever unit you prefer) is above the threshold where you think it poses an unacceptable risk of injury.* The fact that part of the reason the force exceeds that threshold is because you're carrying a pack is irrelevant.
In your example: if 4 Nm isn't enough to hurt you, the should not release at 4 Nm, either when you fall without a pack, or when you're skiing with a pack. If, on the other hand, 4 Nm is enough to hurt you, it should release at 4 Nm, even if the reason you're subjecting yourself to the excessive force is because of the pack.
*The way we choose the release force is slightly complicated, for a few reasons, including the fact that bindings are imperfect measurers of force, and that what we're really afraid of isn't an instantaneous force, but a force over some period of time. The primary determinants are:
- How sturdy the skier is physically. Most narrowly, this is a matter of bone strength, at least if one thinks of a bindings in a sligthly old-fashioned way as designed almost entirely to avoid broken legs. In the old days, there were some binding manufacturers whose manuals used tibia measurements as an input for release settings, rather than weight or height. Of course, now that bindings are all fairly successful at preventing broken legs, the more likely injuries are to ligaments, so the problem is a bit more complicated.
- Because bindings are imperfect: how does the particular user balance the danger of pre-release and non-release? There are basically two sub-reasons here, which align:
* How dangerous is a pre-release? For faster skiers in more dangerous situations, the downside of pre-release is much higher.
* What's the relative prevalance of shocks (high force / short duration) and slow-twists (moderate force / long duration) that the particular skier is likely to experience? For a fast, accomplished skier, shocks are very common and slow twists rare; for a beginner, the opposite is the case.