Secreted effector that acts as an elicitor of hypersensitive response (HR) specifically on plants carrying defense protein R4, through its interaction with this protein (PubMed:18842095, PubMed:19694952). Expression is induced during host plant infection. Oomycete Avh family proteins have a modular structure with an N-terminal signal peptide (SP), an RXLR-dEER domain and a C-terminus with a variable number of modules that consist of W, Y and L motifs after highly conserved residues (PubMed:19694952). The C-ter of Avr4 contains 3 W motifs, 1 Y motif but no L motif. The RXLR-dEER motif functions as a host targeting signal (HTS) (PubMed:18842095). The region containing the W2 motif, in combination with either the W1 or W3 motif, triggers a necrotic response in potato plants carrying the resistance gene R4. Belongs to the RxLR effector family.