May act as a nucleation-promoting factor at the surface of endosomes, where it recruits and activates the Arp2/3 complex to induce actin polymerization, playing a key role in the fission of tubules that serve as transport intermediates during endosome sorting. Interacts (via WHD1 region) with WASHC2C; the interaction is direct (By similarity). Localization to the endosome membrane is mediated via its interaction with WASHC2. WASH genes duplicated to multiple chromosomal ends during primate evolution, with highest copy number reached in humans, whose WASH repertoires probably vary extensively among individuals (PubMed:18159949). It is therefore difficult to determine which gene is functional or not. The telomeric region of chromosome 9p is paralogous to the pericentromeric regions of chromosome 9 as well as to 2q. Paralogous regions contain 7 transcriptional units. Duplicated WASH genes are also present in the Xq/Yq pseudoautosomal region, as well as on chromosome 1 and 15. The chromosome 16 copy seems to be a pseudogene. Belongs to the WASH1 family. The WASH4P N-terminus differs from WASH3P for which it is shown to be required for the WASH complex assembly. Hence is association within the WASH complex is ambiguous. However, WASH4P retains the regions implicated in interaction with WASHC2 and confering in vitro NPF activity. Could be the product of a pseudogene.