When was the 383 4 bolt main engine made?

The 383 cubic inch Chrysler engine with a 4-bolt main bearing cap was not made during a single, specific period. The 4-bolt mains were a *feature* offered on certain versions of the 383, not a defining characteristic of the engine's entire production run.

Chrysler used 4-bolt mains on higher-performance versions of the 383 throughout its production life (roughly the mid-1960s to early 1970s), to increase the strength of the crankshaft. Lower-performance versions continued to use 2-bolt mains. So, to answer when it was *made*, you'd need to specify the model year and the specific engine application (muscle car, truck, etc.) as these details determine whether it had 4-bolt mains.