Phosphoinositides (PIs) are phospholipids that, although a minor component of total cellular lipids, play a myriad of roles in cell biology. PIs function as regulatory molecules, primarily through their ability to recruit or activate proteins at specific intracellular membranes. Specificity is imparted by a plethora of kinases and phosphatases which can variably and combinatorially control the phosphorylation of the PI head group at the 3ˈ, 4ˈ and 5ˈ positions. Consistent with the regulatory role of PIs, PI kinases and phosphatases are frequently dysregulated in cancers. For instance, the PI 3-phosphatase, phosphatase and tensin homolog (PTEN) is one of the most frequently mutated tumour suppressor genes in cancer. The structure of PTEN was first published in 1999 and since then the structures of numerous other human PI 3- and 5- phosphatases have been released. However, despite their significance, no structures of a human PI 4-phosphatases have been solved to date. As a result, the mechanism by which these proteins discriminate between PIs and selectively cleave the 4′ phosphate remains unknown. Herein I present the first structure of a human 4-phosphatase, inositol polyphosphate 4-phosphatase type II B (INPP4B) and use mutagenesis and phosphatase assays to uncover the structural basis for INPP4B 4-phosphatase activity. Further I present the highest resolution structure to date of the INPP4B homologue P-Rex1. Despite containing a canonical dual specificity phosphatase motif P-Rex1 has no reported phosphatase activity. By comparing the structures of INPP4B and P-Rex1 have begun to uncover the structural basis for the differential phosphatase activity of INPP4B and P-Rex1.