This paper presents a comprehensive linear dynamic analysis of a four-degree-of-freedom (4-DOF) lumped mass shear building model, numerically implemented within the OpenSeesPy framework. The numerical model parameters, including mass distribution, stiffness, and damping characteristics, are derived directly from prior experimental testing of physical frame models. The study evaluates the structural response under a statistically significant suite of 50 earthquake ground motion records, carefully selected and scaled to match the Eurocode 8 target spectrum for Soil C and a peak ground acceleration of 0.1 g, reflecting the specific seismic hazard conditions of Osijek, Croatia. Time-history analyses utilizing the Newmark-Beta integration scheme reveal critical insights into the dynamic behaviour, specifically the influence of base flexibility on fundamental period elongation and the distinct response characteristics across different storeys. Statistical processing of the results, visualized through response spectra fractiles and correlation profiles, demonstrates that while lateral displacements exhibit stable first-mode dominance, floor accelerations display high dispersion and significant higher-mode amplification, known as the whipping effect, particularly at the roof level. The findings underscore the limitations of deterministic analyses and highlight the necessity of probabilistic frameworks for accurately assessing seismic demand in low-to-mid-rise structures.