Previously, we identified a GROWTH-REGULATING FACTOR gene family, comprising nine members, which encodes putative transcription factors in Arabidopsis thaliana. The grf1 grf2 grf3 triple mutants produced partially fused cotyledons and developed small leaves due to a reduction in cell numbers. To understand the functional role of another member of this gene family, GRF4, we have now identified a grf4 null mutant and constructed a quadruple mutant by crossing it to the grf triple mutant. The quadruple mutant has much smaller leaves than its parental mutants, with this reduced size again due to fewer cells. Interestingly, the quadruple mutant displays not only a much stronger fusion of cotyledons but also the phenotype of the shoot meristemless mutant. The aberrant cotyledons of the quadruple mutants result from a fusion of cotyledon primordia during embryogenesis. These results suggest that GRF4 is required for both leaf cell proliferation and the embryonic development of cotyledons and the shoot apical meristem.