E4 ubiquitin chain-elongation enzyme specifically involved in polyubiquitin chain assembly. Binds to cdc48 and elongates mono- and diubiquitinated ERAD substrates presented by the ufd1-npl4-cdc48 (UNC) AAA ATPase complex to a chain length of 4 to 6 ubiquitin moieties. Delivers these polyubiquitinated substrates to downstream ERAD components, which target them to the proteasome. Enhances ubiquitination at 'Lys-48', but not at 'Lys-29' of the Ub moiety. Protein modification; protein ubiquitination. The U-box domain is required for the ubiquitin protein ligase activity. Belongs to the ubiquitin conjugation factor E4 family.