Although the generic goals of the fourth workshop are similar to those of the first three, this fourth workshop is different in character because it will be devoted to a specific applied topical area rather than a general one. It is concerned with HIV sequence data, and will have several days devoted to presentations of experimental data and a much shorter time devoted to methodological issues. Studies of HIV sequence variation involve global variation, local variation across individuals, tissue or cell tropism, variation of the transmitted virus between sexual partners or mothers and infants, variation of the quasi-species within an infected individual, and covariation of different sites along the virus. Current methods for analyzing the available information are not fully understood. Combinatorial methods have started to play an important role in the analysis and modeling of HIV sequence data. Among the combinatorial issues involved here are the theory of random trees, the development of measures of variability for trees, the study of measures of distance between trees, and the study of measures of distance between trees, and the study of statistical confidence in a constructed phylogeny. Thus, techniques investigated in our first three workshops will have some bearing on the subject matter of the fourth workshop.