Regulates mitochondrial respiration by decreasing oxygen consumption (PubMed:37198654). Also regulates reactive oxygen species homeostasis (PubMed:37198654). . Core subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I) which catalyzes electron transfer from NADH through the respiratory chain, using ubiquinone as an electron acceptor (PubMed:15250827, PubMed:8344246, PubMed:8644732). Essential for the catalytic activity and assembly of complex I (PubMed:15250827, PubMed:8344246, PubMed:8644732). {ECO:0000269|PubMed:15250827, ECO:0000269|PubMed:8344246, ECO:0000269|PubMed:8644732}. This is the function of MT-ND4 (mitochondrially encoded NADH:ubiquinone oxidoreductase core subunit 4, Ensembl gene identifier ENSG00000198886).