It seems that the NRGY has a metal - Ti - sheet in it, so you'll want a 4.1 mm bit.
As an aside, there are likely some markings on the ski that say that. Most flat skis have some printing that either says 3.6 or 4.1 which refers to the drill bit diameter. If so, there may also be a depth marking, usually - but not always - 9 mm or so.
Skis that have metal in the core construction usually take a 4.1 mm bit, skis that have no metal in the construction generally take a 3.6 mm bit.