The study synthesized a series of 4-substituted analogues (8a-f) of 4-PIOL (5), with substituents at the 4-position of the 3-isoxazolol ring including methyl, ethyl, benzyl, 2-naphthylmethyl, 2-naphthylethyl, and 9-anthracylmethyl. Molecular modeling, GABAA receptor binding assays using [3H]muscimol in rat brain membranes, and whole-cell patch-clamp recordings in cultured cerebral cortical neurons were performed to investigate their interactions with GABAA receptors. Results showed that substitution at the 4-position of 5 was tolerated (unlike that of muscimol (2)), with compounds 8a-c (alkyl/benzyl substituents) having affinities comparable to 5. Notably, the 2-naphthylmethyl-substituted 8d exhibited a 70-fold increase in binding affinity compared to the benzyl analogue 8c, with binding affinity (Ki = 0.049 µM) and antagonistic potency (IC50 = 0.37 µM) comparable to the standard GABAA antagonist gabazine (4). Structural modifications transformed the pharmacological profile of the compounds from the low-efficacy partial agonist activity of 5 to the potent and selective antagonist effect of 8d. These findings supported the hypothesis regarding the different binding modes of muscimol (2) and 5 at the GABAA receptor (with distinct positions of the 4-positions of their 3-isoxazolol rings). Additionally, 8d and related compounds could serve as useful tools for studying GABAA receptor mechanisms.