The Half NDx4 Marumi filter allows bright skies to be easily controlled for dramatic visual effect. The filter is used to control bright/dark contrast by reducing half the shot by 2 stops. It is particulary useful in landscape photography. One half of the ND4 filter is clear while the other is a 4x Neutral Density (2 stops) with a soft boundary between the two. When the ND half of the filter is placed on top, the sky will be darker.
More information about Marumi filters: (Link to Marumi International Website)