Sorry, but I am betting heavily against VTEC being the cause. What is the RPM range for VTEC activation of the high-RPM cam profile on the NSX? 5800-6000 RPM. Simply too high to be related to the dips in question, which are a solid 1000 RPM lower. The torque is climing steadily out of the dip being discussed as it the motor climbs past 5000 RPM.
You can indeed see a very slight dip around the VTEC crossover point though, which is to be expected, but it is at 5800-6000 RPM and is very slight on a NA NSX. On Gerry's high-boost turbo I expect it to be much bigger and thus a noticable dip follows wherever he sets his VTEC crossover point.
Back to the question at hand. Let us consider what other events take place in the NSX motor. At around 4000 RPM the fuel pump goes into high-pressure mode. But just as the VTEC changeover is too high, this is too low to be related to what you see on the graph.
However, right at 4800 RPM the VVIS system opens the buttery fly valves to the second plenum under the main intake manifold, joining both banks of cylinders to the same (larger) intake plenum. The basic theory behind the VVIS system is that keeping the valves closed causes a resonance effect which improves low-end torque, while opening the valves reduces resonance but creates a slight intertia ram effect to give the best performance at high RPM.
I therefore propose you are seeing the motor start to have some air inake issues and then the situation quickly reverse itself once the butterfly valves open and the system is optimized for high-RPM operation.
This also fits with the following fact: You see it on NA NSX motors and see it MAGNIFIED on the BaschBoosted NSXs which maintain the VVIS system, but you do not see it on Comptech supercharged motors which do not keep the VVIS system.
Simple test: Disconnect and cap the vacuum hose to force VVIS to keep the buttefly valves open during a run on a BaschBoosted car and see what happens. At worst you will lose some low-end, but the BB system is designed for high-end anyway. Maybe it would even make sense to permanently lock VVIS open or even remove the butterfly valves on the BaschBoosted cars (not sure how hard removal is). Only dyno testing can say for sure.