Probable tyrosine-protein phosphatase that acts as negative regulator of defense responses against the bacterial pathogen Pseudomonas syringae pv tomato strain DC3000. H2O + O-phospho-L-tyrosyl-[protein] = L-tyrosyl-[protein] + phosphate Highly expressed in flowers and at lower levels in roots, leaves, stems and siliques. Plants overexpressing DSP4 exhibit increased sensitivity to infection by the bacterial pathogen Pseudomonas syringae pv tomato strain DC3000. Belongs to the protein-tyrosine phosphatase family.