Histone methyltransferase that specifically trimethylates 'Lys-20' of histone H4 to form trimethylated histone H4 lysine 20 (H4K20me3) which represents a specific tag for epigenetic transcriptional repression (By similarity). Plays a crucial role in hematopoiesis during embryogenesis by negatively regulating expression of genes related to both primitive and definitive hematopoiesis (PubMed:27377701). L-lysyl(20)-[histone H4] + 3 S-adenosyl-L-methionine = 3 H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl(20)-[histone H4] + 3 S-adenosyl-L-homocysteine Expressed at high levels in the ovary and at lower levels in the fin, testis and brain. Abundantly expressed at early developmental stages but decreases slightly when embryos proceed in development (PubMed:27377701). Expression is strongly detected from 0.25 to 3 hours post-fertilization (hpf) in embryos, but it is only weakly observed at 12 hpf (PubMed:27377701). At 24 and 36 hpf, signals are observed only around the eye with stronger intensities at 24 hpf than 36 hpf (PubMed:27377701). Morpholino knockdown results in embryos which show normal gross morphological development, including of heart and skeletal muscle (PubMed:27377701). However, an increased expression of genes related to both primitive and definitive hematopoiesis is seen (PubMed:27377701). Belongs to the class V-like SAM-binding methyltransferase superfamily.