Plays an essential role in the regulation of viral gene expression by both activating and repressing host RNA polymerase II-mediated transcription (PubMed:30619292). Binds with high affinity to the sequence 5'-ATCGTC-3'. Activates transcription by recruiting a form of the host TFIID to promoters and stabilizing the pre-initiation complex formation. Negatively regulates its own transcription. This immediate early (IE) protein is absolutely necessary for the transition from IE transcription to later viral gene transcription (By similarity). In addition, binds to the host promoters of CXCR3 ligands including CXCL9, CXCL10, and CXCL11 and interacts with TBP to activate their transcription. In turn, mediates CD4+ T-cell migration (PubMed:30619292). Homodimer. Interacts with transcriptional regulator ICP27; this interaction is required for proper incorporation of ICP4 into virions (By similarity). Interacts with host TBP; theis interaction helps the stabilization of the pre-initiation complex on specific promoters (PubMed:30619292). Interacts with host GTF2B. Localizes to the cytoplasm when phosphorylated. The N-terminal and C-terminal domains are required for the transcriptional activation function of ICP4. ADP-ribosylated. The long stretch of Ser is a major site of phosphorylation. Only the phosphorylated forms are capable of interacting with beta or gamma genes. Belongs to the herpesviridae ICP4 family.