The development of chloroplasts in Arabidopsis thaliana requires extensive lipid trafficking between the endoplasmic reticulum (ER) and the plastid. The biosynthetic enzymes for the final steps of chloroplast lipid assembly are associated with the plastid envelope membranes. For example, during biosynthesis of the galactoglycerolipids predominant in photosynthetic membranes, galactosyltransferases associated with these membranes transfer galactosyl residues from UDP-Gal to diacylglycerol. In Arabidopsis, diacylglycerol can be derived from the ER or the plastid. Here, we describe a mutant of Arabidopsis, trigalactosyldiacylglycerol4 (tgd4), in which ER-derived diacylglycerol is not available for galactoglycerolipid biosynthesis. This mutant accumulates diagnostic oligogalactoglycerolipids, hence its name, and triacylglycerol in its tissues. The TGD4 gene encodes a protein that appears to be associated with the ER membranes. Mutant ER microsomes show a decreased transfer of lipids to isolated plastids consistent with in vivo labeling data, indicating a disruption of ER-to-plastid lipid transfer. The complex lipid phenotype of the mutant is similar to that of the tgd1,2,3 mutants disrupted in components of a lipid transporter of the inner plastid envelope membrane. However, unlike the TGD1,2,3 complex, which is proposed to transfer phosphatidic acid through the inner envelope membrane, TGD4 appears to be part of the machinery mediating lipid transfer between the ER and the outer plastid envelope membrane. The extent of direct ER-to-plastid envelope contact sites is not altered in the tgd4 mutant. However, this does not preclude a possible function of TGD4 in those contact sites as a conduit for lipid transfer between the ER and the plastid.