Histone methyltransferase. Methylates 'Lys-4' of histone H3 (H3K4me). H3K4me represents a specific tag for epigenetic transcriptional activation. Acts as a coactivator for estrogen receptor by being recruited by ESR1; thereby activating transcription. {ECO:0000269|PubMed:16603732; ECO:0000269|PubMed:17500065; ECO:0000269|PubMed:17851529}.