Histone methyltransferase that specifically mono- and di-methylates 'Lys-4' of histone H3 in vitro (PubMed:24685137). Does not tri-methylate 'Lys-4' of histone H3 in vitro (PubMed:24685137). Promotes spermatid development and fertility by positively regulating the transcription of spermatocyte-specific genes in primary spermatocytes (PubMed:29702639). Together with spr-5, required for transgenerational fertility (PubMed:24685137). N(6)-methyl-L-lysyl(4)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6),N(6)-dimethyl-L-lysyl(4)-[histone H3] + S-adenosyl-L-homocysteine L-lysyl(4)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6)-methyl-L-lysyl(4)-[histone H3] + S-adenosyl-L-homocysteine In males and hermaphrodites, localizes to foci in the nucleus of spermatocytes, which express the mono- and di-methyl states of 'Lys-4' of histone H3 (PubMed:29702639). These foci are stable nuclear structures with slow diffusion and liquid-like properties (PubMed:29702639). In turn, these foci co-localize with the transcription start sites for major sperm proteins (PubMed:29702639). Expressed in the germline (PubMed:29702639). Predominantly expressed in primary spermatocytes (PubMed:29702639). Also expressed in the oocyte-producing germline of hermaphrodites (PubMed:29702639). Broadly expressed in tissues at the L1 larval stage (PubMed:29702639). In the male germline, first expressed in primary spermatocytes at the L4 larval stage and expression increases as spermatocyte differentiation progresses (PubMed:29702639). RNAi-mediated knockdown partially suppresses the sterility of spr-5 mutants (by101) and results in a slightly increased number of eggs laid as compared to the spr-5 single mutant. Belongs to the class V-like SAM-binding methyltransferase superfamily.