When something in the 3D world is moving in the opposite direction, it can be beneficial to negate that movement:
mouse.x = ( event.clientX / window.innerWidth );
mouse.y = ( event.clientY / window.innerHeight );
// The mouse position now ranges between 0 and 1 for both x and y
// To reverse this direction
mouse.x = 1 - mouse.x;
mouse.y = 1 - mouse.y;
// If the range is from -1 to 1, you can scale it:
mouse.x = mouse.x * 2 - 1; // Initially goes from 0 to 2, then -1 to 1
// Reversing this movement
mouse.x = -mouse.x;
On the other hand, achieving this:
sprite1.position.set( event.clientX *(-.4) , event.clientY *(.4)
might be difficult. It seems like you are converting your screen coordinates in pixels to some world units, as otherwise they may not display correctly. Moving within just the positive range, flipping -4 to 4 could cause it to move offscreen. In order to adjust, consider subtracting an offset like 8.