DNA-dependent RNA polymerase catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Component of RNA polymerase II which synthesizes mRNA precursors and many functional non-coding RNAs. Pol II is the central component of the basal RNA polymerase II transcription machinery. It is composed of mobile elements that move relative to each other. RPB4 is part of a subcomplex with RPB7 that binds to a pocket formed by RPB1, RPB2 and RPB6 at the base of the clamp element. The RBP4-RPB7 subcomplex seems to lock the clamp via RPB7 in the closed conformation thus preventing double-stranded DNA to enter the active site cleft. The RPB4-RPB7 subcomplex binds single-stranded DNA and RNA. The RPB4-RPB7 subcomplex is necessary for promoter-directed transcription initiation but is not required for recruitment of Pol II to active preinitiation complexes and seems to be dispensable for transcription elongation and termination. The RPB4-RPB7 subcomplex recruits FCP1 to Pol II. Involved in DNA repair of damage in the transcribed strand. RPB4 is dispensable under optimal growth conditions, but becomes essential during heat or cold shock and under nutrient depletion. Suppresses the RBP9-mediated transcription-coupled repair (TCR) subpathway of nucleotide excision repair (NER) but facilitates the RAD26-mediated TCR subpathway. Under stress conditions only, involved in mRNA export to the cytoplasm. Involved in mRNA decay. Promotes or enhances the deadenylation process of specific mRNAs and may recruit PAT1 and the LSM1-7 complex to these mRNAs, thus stimulating their decapping and further decay. Component of the RNA polymerase II (Pol II) complex consisting of 12 subunits. RPB4 and RPB7 form a dissociable subcomplex associated with the 10-subunit Pol II core complex. In exponentially proliferating cells, only approximately 20 % of the Pol II complexes contain the RPB4-RPB7 subcomplex. In starving cells, that enter stationary phase, RPB4-RPB7 is associated with Pol II in a stoechiometric manner. The RPB4-RPB7 subcomplex probably associates with TFG1. Interacts with LSM2 and PAT1. Seems to shuttle between nucleus and cytoplasm in a complex with RPB7. Belongs to the eukaryotic RPB4 RNA polymerase subunit family.