Whatever speed you are flying when you enter fixed wing mode.
I said it was an “intelligent flight mode” but that technically isn’t exacty correct. It’s actually a controller mode and you enable it in the controller settings of DJI Go 4. You can be in P-GPS mode or Sport mode for fixed wing mode.
When you have it enabled you click the C1 button to enter fixed wing mode while flying forward faster than some minimum speed which I forget the exact speed. That locks in the speed for fixed wing mode unless you add “throttle” (which is actually the right pitch stick forward) if you let go of the right stick it will slowly revert back to the original speed.