Catalyzes the transfer of the 4'-phosphopantetheine moiety from coenzyme A to a serine residue in the acyl-carrier domain of carboxylic acid reductase Car, thus converting apo-Car to fully active holo-Car. Is probably also responsible for the activation of other proteins with phosphopantetheine attachment sites. apo-[ACP] + CoA = adenosine 3',5'-bisphosphate + H(+) + holo-[ACP] Monomer. Belongs to the P-Pant transferase superfamily.