This book constitutes the thoroughly refereed post-proceedings of the Fourth International Conference on the Quality of Software Architectures, QoSA 2008, held in Medford, MA, USA, in July 2007, in conjunction with the 10th International ACM SIGSOFT Symposium on Component-Based Software Engineering, CBSE 2007, and the ROSATEA 2007 event, investigating the Role of Software Architecture for Testing and Analysis, forming the federated events on Component-Based Software Engineering and Software Architecture, CompArch 2007. The 13 revised full papers presented together with 1 keynote lecture were carefully reviewed and selected from 42 submissions. The papers are organized in topical sections on architectural design and architectural decisions, tracing architectural decisions, architecture evaluation, architecture evolution, architecting process and architectural knowledge.