Probable acetylcholine-gated chloride channel subunit (PubMed:15579462). Does not have ion channel activity alone as a homopentamer, but forms a functional heteropentameric ion channel with acc-1 (PubMed:15579462). May function with lgc-46 to regulate synaptic transmission and synaptic vesicle release in response to acetylcholine in cholinergic motor neurons (PubMed:27782882). Does not form a functional homopentameric ion channel. Forms a functional heteropentameric ion channel composed of acc-1 and acc-4. May interact with acc-2; the interaction does not result in significant heteropentameric ion channel activity. Expressed in all cholinergic motor neurons except for ASJ and RIB neurons. Also expressed in AVF neurons. Grossly normal movement. In addition, in response to induced acetylcholine release, there is prolonged and slow decay of excitory postsynaptic currents which results in increased release of synaptic vesicles during that late phase of synaptic vesicle release in cholinergic motor neurons. Knockout in an lgc-46 gain of function mutant background suppresses the slow locomotion defect in the single lgc-46 gain of function mutant. Knockout together with gain of function mutations in lgc-46 and acr-2 suppresses the convulsion defects in the double lgc-46 and acr-2 double gain of function mutant. Belongs to the ligand-gated ion channel (TC 1.A.9) family.