Catalyzes the activation of fatty acids by CoA to produce an acyl-CoA, the first step in fatty acid metabolism (By similarity). Capable of activating medium-chain fatty acids with a preference for C6-12 fatty acids (By similarity). . This is the function of ACSM4 (acyl-CoA synthetase medium chain family member 4, Ensembl gene identifier ENSG00000215009).