The Android4 HSliderSkin causes the slider track not being filled up to the thumb's position if the slider's thumb is positioned programmatically, i.e. positioned not by user interaction but by setting the value property of the HSlider's instance in code.
Quick fix by adding the following code lines to the set hostComponent function in HSliderSkin.as:
if (_hostComponent) _hostComponent.removeEventListener(FlexEvent.VALUE_COMMIT, thumbPositionChanged_handler);
if (_hostComponent) _hostComponent.addEventListener(FlexEvent.VALUE_COMMIT, thumbPositionChanged_handler);