Three-dimensional coherent islands formed during the highly strained growth of In(0.3)G(0.7)As on GaAs(001) are studied by scanning tunneling microscopy. High-resolution images evidence two different types of surface reconstructions between the top and the bottom of the islands. While a 2x4 GaAs(001)-like reconstruction is observed on the wetting layer, the top layer exhibits the (2x4)alpha 2 phase, which is characteristic of the InAs(001) reconstructed surface. This is the consequence of In surface segregation leading to the formation of a monolayer of InAs at the island top. Finally, photoluminescence experiments exemplify the effect of segregation on the InxGa1-xAs/GaAs quantum box optical properties.