The plant laccase gene family has 17 genes in Arabidopsis, but their functions have not been clearly demonstrated. We analyzed the function of AtLAC4 by overexpressing it in wild-type plants. GUS staining revealed that AtLAC4 strongly expressed in vascular tissues and specifically in the leaf hydathodes. AtLAC4-overexpressor plants were dwarf and had shorter leaf petioles, both caused by increased lignin content and secondary wall thickness. The expression of AtLAC4 was induced significantly by abscisic acid (ABA), and AtLAC4-overexpressor plants were sensitive to exogenous ABA. Drought tolerance was enhanced in AtLAC4 overexpression than in wild-type plants. AtLAC4 may play important roles in regulating plant growth, and responding to abiotic stress.