WCOP'99 is the fourth event in a series of highly successful workshops, which took place in conjunction with every ECOOP since 1996, focusing on the important field of component-oriented programming (COP). COP has been described as the natural extension of object-oriented programming to the realm of independently extensible systems. Several important approaches have emerged over the recent years, including CORBA, COM (COM+, DCOM, ActiveX, DirectX, ...), JavaBeans. A component is not an object, but provides the resources to instantiate objects. Often, a single component will provide interfaces to several closely related classes. Hence, COP is about architecure and packaging, besides interoperation between objects. After WCOP'96 focused on the fundamental terminology of COP, the subsequent workshops expanded into the many re- lated facets of component software. WCOP'99 shall emphasis architectural design and construction of component-based systems beyond ad-hoc reuse of unrelated components. To enable lively and productive discussions, the workshop will be limited to 25 participants. Depending on the actually submitted positions papers, the workshop will be organized into three or four subsequent mini-sessions, each initiated by a presentation of two or three selected positions and followed by discussions. Instead of splitting the workshop into task forces, it is intended to provoke lively discussion by preparing lists of critical questions and some, perhaps provocative, statements (to be used on demand). Position papers will be formally reviewed, each by at least two independent reviewers. As an incentive for submission of high quality statements, the best position statements will be combined with transcripts of workshop results and published.