Plays an essential role in the regulation of viral gene expression by both activating and repressing host RNA polymerase II-mediated transcription. 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. Forms homodimers (PubMed:2991559, PubMed:28505309). Interacts with transcriptional regulator ICP27; this interaction is required for proper incorporation of ICP4 into virions (PubMed:8995681). Interacts with host TBP and host TAF1; these interactions help the stabilization of the pre-nitiation complex on specific promoters (PubMed:8649420). Interacts with host GTF2B (PubMed:8392607). 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.