The fourth volume of the Sora no Otoshimono manga was first released on January 26th, 2009 by the Kadokawa Shoten Publishing Corporation. This volume introduces the wicked Harpies, two of the Man of Synapse's Angeloid servants that use deception in order to make the hapless Nymph get Ikaros for them so that they can complete their master's mission and dispose of her for her failures.
Chapter Thirteen: Multiply!!
Chapter Fourteen: Smile Attack!!
Chapter Fifteen: Toy!!
Chapter 15.5: The North Wind and the Sun (Special Angeloid-focused Chapter)
Chapter Sixteen: Orders