Poorly processive, error-prone DNA polymerase involved in translesion repair and untargeted mutagenesis (PubMed:10488344, PubMed:10801133). Copies undamaged DNA at stalled replication forks, which arise in vivo from mismatched or misaligned primer ends. These misaligned primers can be extended by Pol IV. Exhibits no 3'-5' exonuclease (proofreading) activity (PubMed:10488344). Overexpression of Pol IV results in increased frameshift mutagenesis. It is required for stationary-phase adaptive mutation, which provides the bacterium with flexibility in dealing with environmental stress, enhancing long-term survival and evolutionary fitness. Not seen to be involved in translesion snythesis even when stimulated by the beta slding-clamp and clamp-loading complex, which do however increase non-targeted DNA polymerase efficiency 3,000-fold, may be due to targeting to stalled replication forks on nondamaged DNA (PubMed:10801133, PubMed:16168375). Involved in translesional synthesis, in conjunction with the beta clamp from PolIII (PubMed:14592985, PubMed:14729336). a 2'-deoxyribonucleoside 5'-triphosphate + DNA(n) = diphosphate + DNA(n+1) Binds 2 magnesium ions per subunit. Monomer. Interacts with beta sliding clamp, which confers increased processivity (PubMed:14592985, PubMed:14729336,PubMed:16168375). By SOS response. A member of the dinB-yafNOP operon (PubMed:12813093). Induced by hydroxyurea (PubMed:20005847). The catalytic core consists of fingers, palm and thumb subdomains, but the fingers and thumb subdomains are much smaller than in high-fidelity polymerases; residues from five sequence motifs of the Y-family cluster around an active site cleft that can accommodate DNA and nucleotide substrates with relaxed geometric constraints, with consequently higher rates of misincorporation and low processivity. It lacks the O helices present in high-fidelity DNA polymerases in the fingers domain (By similarity). Belongs to the DNA polymerase type-Y family.