Catalyzes the synthesis of 4-coumaroyl-homoserine lactone, a quorum-sensing (QS) autoinducer molecule which binds to RpaR transcriptional regulator to regulate expression of QS-dependent genes. 4-coumaroyl-CoA + S-adenosyl-L-methionine = CoA + H(+) + N-(4-coumaroyl)-L-homoserine lactone + S-methyl-5'-thioadenosine Repressed by RpaR in the absence of 4-coumaroyl-homoserine lactone. Induced by 4-coumaroyl-homoserine lactone. Belongs to the autoinducer synthase family.