Double-stranded RNA-binding protein involved in RNA-mediated post-transcriptional gene silencing (PTGS). Functions in the trans-acting small interfering RNAs (ta-siRNAs) biogenesis by binding and assisting DICER-LIKE 4 (DCL4). Required for DCL4 activity. Required for the 21 nucleotide ta-siRNAs production of the TAS3 transcript in leaves but not in flowers. Plays an important role in silencing RNA of both DNA and RNA viruses. Involved with argonaute 7 (AGO7) and RDR6 in turnip crinkle virus (TCV) silencing. May not be directly involved in viral siRNA production. May stabilize the 21 nucleotide viral siRNAs and deliver them to the RISC complex. Targeted by the viral silencing suppressor (VSR) transactivator/viroplasmin (TAV) protein of the cauliflower mosaic virus (CaMV) that inactivates DRB4 function in RNA silencing. Probably not involved in the guide strand selection from RNA duplexes. Involved in leaf morphology through its function in ta-siRNA-mediated silencing. Heterodimer with DRB1 or DRB5. Interacts with DCL4 and cauliflower mosaic virus (CaMV) transactivator/viroplasmin protein. Interaction with CaMV transactivator/viroplasmin protein inhibits RNA silencing ability of DRB4. Expressed in roots, leaf vasculature, shoot apical meristem (SAM) and developing anthers. Elongated and downwardly curled rosette leaves. In old plants, rosette and cauline leaves and flowers change to red color. Increased levels of ARF3 and ARF4 transcripts, targeted by TAS1 and TAS3, respectively.