The function of SPSB4 (splA/ryanodine receptor domain and SOCS box containing 4, ENSG00000175093) is as follows. Substrate recognition component of a SCF-like ECS (Elongin BC-CUL2/5-SOCS-box protein) E3 ubiquitin-protein ligase complex which mediates the ubiquitination and subsequent proteasomal degradation of target proteins (PubMed:15601820, PubMed:21199876). Negatively regulates nitric oxide (NO) production and limits cellular toxicity in activated macrophages by mediating the ubiquitination and proteasomal degradation of NOS2 (PubMed:21199876). Acts as a bridge which links NOS2 with the ECS E3 ubiquitin ligase complex components ELOC and CUL5 (PubMed:21199876). Diminishes EphB2-dependent cell repulsive responses by mediating the ubiquitination and degradation of EphB2/CTF2 (PubMed:28931592). Regulates cellular clock function by mediating the ubiquitin/proteasome-dependent degradation of the circadian transcriptional repressor NR1D1 (PubMed:26392558). {ECO:0000269|PubMed:15601820, ECO:0000269|PubMed:21199876, ECO:0000269|PubMed:26392558, ECO:0000269|PubMed:28931592}.