Involved in the phenylpropanoid metabolism by mediating the activation of a number of hydroxycinnamates for the biosynthesis of monolignols and other phenolic secondary metabolites (PubMed:21807887, PubMed:23246835). Catalyzes the formation of CoA esters of cinnamate, 4-coumarate, caffeate and ferulate (PubMed:21807887, PubMed:23246835). Is more efficient with substrates in the following order: ferulate > 4-coumarate > caffeate > cinnamate (PubMed:21807887). Possesses very high activity compared to 4CL1, 4CL2, 4CL4 and 4CL5 (PubMed:21807887). Cannot convert sinapate to its corresponding CoA ester (PubMed:21807887, PubMed:23246835). May play a role in the synthesis of lignin as well as other phenolic compounds (PubMed:21807887). (E)-4-coumarate + ATP + CoA = (E)-4-coumaroyl-CoA + AMP + diphosphate kcat is 10.92 min(-1) with cinnamate as substrate (PubMed:21807887). kcat is 17.1 min(-1) with 4-coumarate as substrate (PubMed:21807887). kcat is 15.3 min(-1) with caffeate as substrate (PubMed:21807887). kcat is 17.94 min(-1) with ferulate as substrate (PubMed:21807887). Phytoalexin biosynthesis; 3,4',5-trihydroxystilbene biosynthesis; 3,4',5-trihydroxystilbene from trans-4-coumarate: step 1/2. Expressed in root exodermis and epidermis cells, stem vascular cells, leaf developing vascular bundle cells and parenchyma cells, lemma, palea, stamens and pistil. Induced by wounding (PubMed:23246835). Down-regulated by UV irradiation (PubMed:23246835). Both substrate-binding domains (SBD1 and SBD2) are involved in the substrate recognition, and are sufficient to confer the substrate specificity. Plants silencing 4CL3 exhibit significant lignin reduction, reduced plant height, decreased panicle fertility and abnormal anther development. Belongs to the ATP-dependent AMP-binding enzyme family.