Since 1979, the U.S. Army Research Laboratory* has been developing and distributing the Ballistic Research Laboratory - Computer-Aided Design (BRL-CAD) three-dimensional (3-D) solid modeling package to support combat vehicle vulnerability studies and various other military and industrial applications. The software, which is now in its third generation, includes a large collection of tools and utilities, including an interactive geometry editor, raytracing and generic framebuffer libraries, network-distributed image-processing/signal-processing capabilities, and an embedded scripting language. In support of the package, a multivolume tutorial series is being written to assist users with the many features and functionality of BRL-CAD. Three volumes have been published thus far. Volume I provides an overview of the package contents and installation (Butler and Edwards, 2002). Volume II addresses the basic functionality of the package's Multi-Device Geometry Editor (MGED) and offers a comprehensive list of the user commands available (Butler et al., 2001). Volume III discusses the modeling process as well as principles and techniques to help maximize BRL-CAD's effectiveness (Butler et al., 2003). The purpose of Volume IV is to discuss issues of compatibility and conversion between the BRL-CAD geometry file format and the formats of various other computer-aided design (CAD), computer-aided manufacturing (CAM), and computer-aided engineering (CAE) packages. Conversion is increasingly important for BRL-CAD users who must interact with a growing number of Government and commercial organizations involved in the research, development, testing, and evaluation of today's combat systems. Army Research Laboratory. ARL-SR-121.