To two-dimensional or axisymmetric water wave problems, many finite element techniques proposed hitherto have been successfully applied. In case of truly three-dimensional analysis, however, much attention should be payed, because most of them generally have a tendency to require a large digital computer and consume more computer time and cost. To overcome these difficulties, the auther has developed a new solution method based on the extended Haskind's relation in the third report and succeeded in achieving high computational efficiency by (1) the reduction of the domain to be analyzed by introducing a fictitious boundary near the body and (2) the decomposition of the original problem into a series of fictitious elementary problems without coupling, which leads to a series of discretized algebraic equations with the same real symmetric and banded matrix and (3) the reconstruction of the original solution from the numerical elementary solutions on the above relation. In the present paper, an alternative variational formulation for fictitious elementary wave potentials is presented on the basis of the extended Sommerfeld's type radiation condition. For vertically truncated cylinders with arbitrary sections, the flow field can be subdivided into some vertically cylindrical subdomains by fictitious boundaries and each velocity potential there can be expressed by combining vertically orthogonal eigenfunctions with horizontally piecewise quadratic interpolation functions under the continuity of the velocity potential and its normal derivative on each fictitious boundary. As the result, the vertical orthogonality reduces the above functional in more tractable form and achieves a considerable saving of numerical effort. The calculated results are shown to compare favorably with the existing analytical solutions. Conclusions are that these procedures are powerful working tools for practical offshore hydrodynamic investigations.