Histone methyltransferase that catalyzes methyl group transfer from S-adenosyl-L-methionine to the epsilon-amino group of 'Lys-4' of histone H3 (H3K4). Part of chromatin remodeling machinery predominantly forms H3K4me1 methylation marks at active chromatin sites where transcription and DNA repair take place. Acts as a coactivator for estrogen receptor by being recruited by ESR1, thereby activating transcription. L-lysyl(4)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6)-methyl-L-lysyl(4)-[histone H3] + S-adenosyl-L-homocysteine Component of the MLL2 complex (also named ASCOM complex), at least composed of catalytic subunit KMT2D/MLL2, ASH2L, RBBP5, WDR5, NCOA6, DPY30, KDM6A, PAXIP1/PTIP, PAGR1 and alpha- and beta-tubulin (PubMed:21335234). Forms a core complex with the evolutionary conserved subcomplex WRAD composed of WDR5, RBBP5, ASH2L/ASH2 and DPY30 subunits; WRAD differentially stimulates the methyltransferase activity. Interacts with ESR1; interaction is direct. Interacts (via WIN motif) with WDR5 (By similarity). LXXLL motifs 5 and 6 are essential for the association with ESR1 nuclear receptor. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. TRX/MLL subfamily. Human protein KMT2B/MLL4 was first named MLL2 (see AC Q9UMN6). Thus, also mouse Mll4 is often referred to as Mll2 and vice versa in the literature.