Required for RNA-mediated gene silencing (RNAi). Binds to short RNAs such as microRNAs (miRNAs) and represses the translation of mRNAs which are complementary to them. Lacks endonuclease activity and does not appear to cleave target mRNAs. Also required for RNA-directed transcription and replication of the human hapatitis delta virus (HDV). Interacts with EIF4B, IMP8, PRMT5, TNRC6A and TNRC6B (PubMed:19167051). Interacts with ZFP36 (PubMed:15766526). Ubiquitinated on surface-exposed lysines by a SCF-like E3 ubiquitin-protein ligase complex containing ZSWIM8 during target-directed microRNA degradation (TDMD), a process that mediates degradation of microRNAs (miRNAs). Ubiquitination by the SCF-like E3 ubiquitin-protein ligase complex containing ZSWIM8 leads to its subsequent degradation, thereby exposing miRNAs for degradation. ZSWIM8 recognizes and binds AGO4 when it is engaged with a TDMD target. Belongs to the argonaute family. Ago subfamily.