Serotonin 5-HT4 receptors (5-HT4Rs) have emerged as potential therapeutic targets in neuropsychiatric and neurodegenerative disorders by modulating circuits that shape mood, cognition, and motor functions. Ligands for 5-HT4Rs can modify dopamine (DA) and acetylcholine (ACh) transmission, but mechanisms and circuits have not been fully resolved. Some 5-HT4R agonists have been suggested to have effects that include the inhibition of acetylcholinesterase (AChE), raising the speculation that 5-HT4R ligands might modulate ACh and DA through this action. Here, we investigated the impact of RS67333, a partial 5-HT4R agonist, on DA and ACh release dynamics in the striatum detected ex vivo in mouse brain slices using fast-scan cyclic voltammetry (FCV) and genetically encoded ACh sensor GRABACh3.0, respectively. We found that RS67333 significantly modulated electrically evoked DA release in the dorsolateral striatum (DLS) and nucleus accumbens core, effects that were abolished by a nicotinic receptor (nAChR) antagonist. In parallel, RS67333 altered evoked ACh signals by extending extracellular ACh lifetime, and correspondingly, RS67333 was found to inhibit striatal AChE enzymatic activity. By contrast, BIMU8, an alternative 5-HT4R ligand that did not inhibit striatal AChE, had no effect on the evoked striatal ACh or DA release. These findings indicate that RS67333 modulates striatal ACh transmission, which shapes downstream regulation of DA release by nAChRs, not through 5-HT4Rs but through AChE inhibition. These findings emphasize the caution due in attributing functions to 5-HT4Rs but also highlight an alternative pharmacological profile of some purported 5-HT4R ligands as AChE inhibitors of potential utility for treating ACh/DA disorders.