The D4 Uracil-DNA glycosylase component of the E9-A20-D4 polymerase holoenzyme with a 1:1:1 stoichiometry. Also refer to UniProt https://www.uniprot.org/uniprot/P20509 (E9 component) and UniProt https://www.uniprot.org/uniprot/P20995 (A20 component).