RNA polymerase IV is an enzyme which synthesizes small interfering RNA (siRNA) in plants. Polymerase IV is specific to plants genomes and is required for the synthesis of over 90% of all siRNA. 
RNA polymerase silences the transposons and repetitive DNA in the siRNA pathway. The siRNA plays a major role in defending the genome against the invading viruses and transposable elements by RNA directed DNA methylation. 
Polymerase IV and ROS1 demethylase unlocks and recondenses the 5S rDNA chromatin, which is present in seed and used for the development of adult features in plants.  Polymerase IV is involved in setting the methylation patterns in the 5S genes during plant maturation. 
In Arabidopsis polymerase IV works with binding protein DCL3 and a RNA polymerase II RDR2 in a silencing pathway which Polymerase IV would produce RNA, which is changed to dsRNA by RDR2 then converted to siRNA by DCL3.