Anti-sigma factor for SigI4. Negatively regulates SigI4 activity through direct interaction. Binding of the polysaccharide substrate to the extracellular C-terminal sensing domain of RsgI4 may induce a conformational change in its N-terminal cytoplasmic region, leading to the release and activation of SigI4. Interacts (via RsgI N-terminal anti-sigma domain) with SigI4. Up-regulated in pretreated yellow poplar (PYP)-grown cells. The CBM3 domain binds to cellulose.