This is an interesting discussion. I am not sure where the idea that approximately -4 degree nose angle is somehow optimal originated. I saw Brodie Smith mentioned it in one of his TechDisc videos, but without any justification. The wind will not flip a disc up or down during the flight. Since the disc is spinning, any torque is translated into turn/fade instead. However, the change of direction of the lift and drag caused by the nose angle will make the disc change its trajectory. The extreme version of this is an air bounce, where throwing the disc on a downward trajectory with very high nose angle eventually will change the trajectory to go upwards (or vice versa, the reverse air bounce, as
@sidewinder22 mentioned
)
To add some quantitative data to the discussion, here is how the aerodynamic coefficients for a disc typically change around 0 degrees angle of attack (nose angle at the point of release). This data is taken from computational fluid dynamics simulations I have performed, and the trends are similar across a large range of discs.
View attachment 336192
So having nose down makes the disc behave more understable (lower moment coefficient) and reduces the lift, while the drag more or less stays the same (even a slight reduction here, but it varies between discs).
If we plug this into a trajectory simulator we can compare the flights. This is not perfect, of course, but the benefit of a simulator is that you are able to really isolate the effect of a single parameter. Here are two simulated throws, where the one with nose angle down also has a higher hyzer angle to compensate for the lower initial moment coefficient:
View attachment 336193
So why does the lower nose angle go further, even though we have reduced the lift? The answer is that by reducing the lift while maintaining the momentum and not increasing drag, we allow the disc to push more forward instead of rising up. This also makes the fade towards the end of the flight less pronounced, as you don't get as severe angle of attack towards the end of the flight. So I believe a slightly negative nose angle is beneficial, maybe especially when throwing faster, overstable discs.