Catalyzes the conversion of 4-hydroxy-tetrahydrodipicolinate (HTPA) to tetrahydrodipicolinate. Can use both NADH and NADPH as a reductant, with NADH being twice as effective as NADPH. (S)-2,3,4,5-tetrahydrodipicolinate + H2O + NAD(+) = (2S,4S)-4-hydroxy-2,3,4,5-tetrahydrodipicolinate + H(+) + NADH (S)-2,3,4,5-tetrahydrodipicolinate + H2O + NADP(+) = (2S,4S)-4-hydroxy-2,3,4,5-tetrahydrodipicolinate + H(+) + NADPH Is inhibited by 2,6-pyridine dicarboxylate (2,6-PDC or picolinate). Amino-acid biosynthesis; L-lysine biosynthesis via DAP pathway; (S)-tetrahydrodipicolinate from L-aspartate: step 4/4. Homotetramer. Expression of dapB is up-regulated by ArgP and is repressed by lysine that prevents the binding of the ArgP activator. Thus, ArgP contributes to enhanced transcription of dapB when lysine becomes limiting. Belongs to the DapB family. Was originally thought to be a dihydrodipicolinate reductase (DHDPR), catalyzing the conversion of dihydrodipicolinate to tetrahydrodipicolinate. However, it was shown that the substrate of the enzymatic reaction is not dihydrodipicolinate (DHDP) but in fact (2S,4S)-4-hydroxy-2,3,4,5-tetrahydrodipicolinic acid (HTPA), the product released by the DapA-catalyzed reaction (PubMed:20503968).