E4 ubiquitin ligase that extends E3-dependent Ub chains; catalyzes K48-linked multi-monoubiquitination of Ufd4p-assembled, K29-linked Ub chains prior to substrate degradation; binds and converts C-terminally extended Ub (Ubi4p) into an unanchored chain to mark client proteins for stress-induced degradation; role in Ub-fusion degradation, the OLE pathway, and the ERAD pathway; U-box family member; human homologs, UBE4A and UBE4B, implicated in neurological disease and tumor development