Histone methyltransferase that selectively methylates 'Lys-4' of dimethylated histone H3 (H3K4me2) to produce trimethylated 'Lys-4' histone H3 (H3K4me3). May play a role in epigenetic regulation of gene expression by defining an active chromatin state. . This is the function of PRDM7 (PR/SET domain 7, ENSG00000126856).