We could draw a resonance form with the pi electrons shifting, placing a positive charge on C-4, to which a $\ce{H+}$ along with 2 electrons are added. Thus we get an NADH molecule as shown in the picture.
If this is how the reaction happens, then why doesn't the hydrogen get added to C-2 (or C-6 which is equivalent)? There's also a resonance form where C-2 has a positive charge and could accept a hydride ion. Why doesn't this happen?
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.
