Actin-depolymerizing protein. Severs actin filaments (F-actin) and binds to actin monomers (PubMed:19346440, PubMed:22010035). Contributes to the stochastic dynamic turnover of actin filaments (PubMed:22010035). Binds monomeric actin (G-actin) with a marked preference for the ADP-loaded form and inhibits the rate of nucleotide exchange on G-actin. Involved in resistance triggered by the effector AvrPphB of Pseudomonas syringae pv tomato (Pst). May modulate the AvrPphB-RPS5-mediated defense signal transduction pathway (PubMed:19346440). During AvrPphB-triggered resistance signaling pathway, involved in the control of MPK3 and MPK6 activation, via the coordinated regulation of actin cytoskeletal dynamics and RPS5 resistance gene transcription (PubMed:23144618). During innate immune response triggered by the bacterial elf26 peptide, the inhibition of ADF4 regulates actin dynamics in order to execute key events associated with pattern-triggered immunity (PTI), such as cell wall fortification and transcriptional activation of defense gene markers (PubMed:24464292). By the root-knot nematode Meloidogyne incognita. Phosphorylation at Ser-6 is required for resistance to Pseudomonas syringae pv tomato AvrPphB. Increased root length under light-grown conditions. Increased length of hypocotyls under dark-grown conditions. Altered architecture of the actin cytoskeleton in hypocotyl cells (PubMed:22010035). Compromised resistance in response to Pseudomonas syringae pv tomato expressing AvrPphB (PubMed:19346440). Belongs to the actin-binding proteins ADF family.