Probable substrate-recognition component of a SCF-like ECS (Elongin-Cullin-SOCS-box protein) E3 ubiquitin-protein ligase complex which mediates the ubiquitination and subsequent proteasomal degradation of target proteins. Promotes differentiation and maturation of the vascular lineage by an oxygen-dependent mechanism. Protein modification; protein ubiquitination. Interacts with HIF1AN. Component of an ECS (Elongin BC-CUL2/5-SOCS-box protein) E3 ubiquitin-protein ligase complex formed of CUL2 or CUL5, Elongin BC (ELOB and ELOC), RBX1 and ASB4. Highest expression detected in testis, with some expression detected in ovary and heart. Not detected in lung, kidney, liver, spleen and bone marrow. Differentially expressed in the vascular lineage during embryonic stem (ES) cell differentiation, with expression increasing when the cells differentiate. At 7.5 dpc, no remarkable expression is detected. At 9.5 dpc, expressed in intersomitic vessels, dorsal aorta, forelimb buds, allantois/umbilical vessels, vitelline vessels, septum transversum, proepicardium, capillary plexi of the head and branchial arches, endocardium, yolk sac vasculature and placenta. At 10.5 dpc, expressed in forelimb and hind limb buds, intersomitic vessels, peripheral liver cells and umbilical vessels, with expression in the capillary plexi of the head and branchial arches no longer detected. At 11.5 dpc, high expression levels are limited to forelimbs and hind limbs and the most caudal intersomitic vessels. The SOCS box domain mediates the interaction with the Elongin BC complex, an adapter module in different E3 ubiquitin-protein ligase complexes. Hydroxylation at Asn-246 by HIF1AN may provide an oxygen-dependent regulation mechanism for the function of ASB4 in promoting vascular differentiation. Belongs to the ankyrin SOCS box (ASB) family.