The w in vec4(x, y, z, w) is used for clipping, and plays its part while linear algebra transformations are applied to the position.
By default, this should be set to 1.0.
See here for some more info: http://web.archive.org/web/20160408103910/http://iphonedevelopment.blogspot.com/2010/11/opengl-es-20-for-iOS-chapter-4.html