Shield 4 is an outdoor level, on top of the castle, where you have to make your way past platforms that are not the same every time, and get to the roof, and take the Shield avoiding the Clouds. It is the fourth and final level on the Shield Level Path, in Dark Castle. It has shown up in Dark Castle, and Color Dark Castle, and will show up in Return to Dark Castle.
Levels on the Path
Main article: Shield Level Path