Transcriptional activator. Binds to the interferon-stimulated response element (ISRE) of the MHC class I promoter. Binds the immunoglobulin lambda light chain enhancer, together with PU.1. Probably plays a role in ISRE-targeted signal transduction mechanisms specific to lymphoid cells. Involved in CD8(+) dendritic cell differentiation by forming a complex with the BATF-JUNB heterodimer in immune cells, leading to recognition of AICE sequence (5'-TGAnTCA/GAAA-3'), an immune-specific regulatory element, followed by cooperative binding of BATF and IRF4 and activation of genes. Interacts with SPIB and DEF6 (By similarity). Interacts with the BATF-JUNB heterodimer. Interacts with BATF (via bZIP domain); the interaction is direct. Directly interacts with NLRP3 in the nucleus of Th2 cells; this interaction enhances IRF4 ability to bind to the IL4 promoter and is required for optimal IRF4-dependent IL4 transcription (PubMed:26098997). Lymphoid cells. Not induced by interferons. Phosphorylation by ROCK2 regulates IL-17 and IL-21 production. Belongs to the IRF family.