This study evaluates the performance of Nigeria’s Fourth National Development Plan (1981–1985) with reference to its long-term socio-economic impact forty years after its initiation. The specific objectives were to examine the Plan’s stated objectives and strategic priorities, assess its implementation across critical sectors—agriculture, industry, infrastructure, employment, and regional development—and evaluate its long-term legacy on Nigeria’s development trajectory. The study adopted a descriptive survey research design. The population comprised policymakers, development experts, academics, and public officials, from which a sample size of 222 was selected using Taro Yamane’s formula. Data were collected through structured questionnaires and interviews and analyzed using descriptive statistics and the Chi-square test. Findings revealed a statistically significant relationship between the Plan’s objectives and the policy intentions behind its formulation. Additionally, there was a significant association between its implementation and the performance of key sectors, as well as long-term impacts on Nigeria’s socio-economic development. However, the study found that corruption, policy inconsistency, and weak institutional capacity hindered full realization of the Plan’s goals. The study concludes that while the Fourth Plan had well-structured objectives, weak implementation diluted its outcomes. It recommends strengthening institutional monitoring, ensuring policy continuity, and aligning future plans with Nigeria’s fiscal realities for improved national development performance.