This is the fourth part of a series on the butterflies of Colorado. The book is written in the same style and format as the original Butterflies of Colorado published by the incomparable F.M. Brown in concert with Donald Eff and Reverend Bernard Rotger in 1957. This is a completely new book, but the author makes comparisons with Brown et al.'s original work, pointing out corrections, additions, and emendations. The book is written in a warm personal style. In the first section 156 species and all Colorado subspecies are discussed with at least a page dedicated to each species and its described subspecies. Each species' variation within Colorado, and outside where relevant, is discussed and the problems of name application, species definition, and type localities is also discussed. Neosatyrium Fisher is described as a new subgenus, Euphilotes stanfordorum Opler and Warren and Euphilotes ancilla barnesi are described as new, Plebejus cotundra Scott is a new combination recognized as a full species; while Plebejus scudderii sublivens Nabokov is another new combination.