Involved in the first step of peroxisomal beta-oxidation by catalyzing the desaturation of fatty acid-derived side chains of ascaroside pheromones, which regulates development and behavior (PubMed:29537254, PubMed:29863473). Specifically, shortens ascarosides with a 9-carbon side chain (asc-C9) and, in association with acox-1.1, may contribute to the shortening of ascarosides with a 11-carbon side chain (asc-C11) (PubMed:29537254, PubMed:29863473). May contribute to the production of indol-3-carbonyl(IC)-ascarosides in association with acox-1.1 and acox-3 (PubMed:29863473). asc-C9-CoA + O2 = asc-DeltaC9-CoA + H2O2 Activated by ATP (By similarity). ATP binding leads to a conformational change that promotes FAD cofactor binding and enzyme activity (By similarity). ATP binding likely occurs during acox-1.4 folding and/or dimer formation (By similarity). Lipid metabolism; peroxisomal fatty acid beta-oxidation. Homodimer. Belongs to the acyl-CoA oxidase family.