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/p97 (UNC) AAA ATPase complex to a chain length of 4 to 6 ubiquitin moieties. Delivers these polyubiquitinated substrates to RAD23 and DSK2, which target them to the proteasome. Has E3 ubiquitin-protein ligase activity, accepting ubiquitin from its cognate E2 ubiquitin-conjugating enzyme UBC4. Enhances ubiquitination at 'Lys-48', but not at 'Lys-29' of the Ub moiety. Promotes ubiquitin chain elongation at 'Lys-48' on the DOA10 substrate PEX29. Also involved in the proteolytic processing of the ER-bound transcription factor SPT23. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Interacts with CDC48. Interacts with the ubiquitin-like domain of RAD23 and DSK2. Interacts with PEX29. The U-box domain is required for the ubiquitin protein ligase activity. Present with 2600 molecules/cell in log phase SD medium. Belongs to the ubiquitin conjugation factor E4 family.