mouko kouhazan kakuda chouchu

This move requires "Hit Timing", also known as a "Just Frame", input.

That is, you enter the the moment the makes contact (guard or hit) with the opponent. So trying to do it as fast as possible will always result in failure. From memory, VF4 Evo's training mode has a slow motion feature, so maybe try performing it slowed down so you'll have an easier time visually/aurally timing the input. Once you're comfortable with that, go back to normal speed.

You can also try searching this forum for other tips on this move, also known as the Single Palm into Dashing Elbow (or variants of).