Why is NAD+ regioselectively reduced at C-4 to form NADH?
Living organisms utilize protein based enzymes to catalyze otherwise unfavorable reactions under very mild conditions. They achieve this by bringing the substrates together in a very specific manner. Dehydrogenases are a class of oxidoreductases that oxidize substrates using NAD$^+$. Here is the active site of lactate dehydrogenase:



As you can see, carbon-4 of the nicotinamide ring is oriented towards the substrate. The orientation of the molecules is why carbon-4 is selectively reduced.