This study aimed to evaluate the fit of the Three-Parameter Logistic Model (3PLM) and Four-Parameter Logistic Model (4PLM) models of Item Response Theory (IRT) for an achievement test in a general course. Test data from 1168 second-year students at the Faculty of Education, Zagazig University, were analyzed using item fit indices, and overall model fit indices. person fit was also examined using the zh statistic. Furthermore, person ability was estimated using Maximum Likelihood Estimation (MLE), Maximum A Posteriori (MAP), and Expected A Posteriori (EAP) methods, with a comparison of measurement precision. Finally, the overall Test Information Function (TIF), Standard Error of Measurement (SEM), and empirical and marginal reliability were assessed. Results indicated that the 3PL model achieved a better fit for individual items and the overall model compared to the 4PL model, excelling in most global fit indices (AIC, BIC, SRMSR, logLik). Although both models showed high fit for individual response patterns, the 4PL model identified slightly fewer individuals with misfit response patterns. For person ability estimation, both MAP and EAP methods demonstrated very high and identical measurement precision for both models, outperforming the unstable MLE method. It was also observed that the 4PL model tended to estimate a higher average person ability compared to the 3PL model. Regarding test information and reliability, the 3PL model provided a higher test information function across a wider range of ability and achieved significantly higher empirical and marginal reliability values than the 4PL model. These findings suggest that the added complexity of the 4PL model did not yield substantial improvements to justify its use for this test, and in fact, negatively impacted reliability estimates.