If someone says that their Chevy has two 4's underneath the hood, what do they mean is under the hood?

Or two four barrel carburetors.

Unless they are a late model teenager living in Mom's basement they mean a high performance V8 engine with two four barrel carburetors, generally mounted front to rear on a low rise intake manifold. If mounted on a hi-ram they would probably say "tunnel ram" and if mounted on each side of the engine they would probably say "cross ram". If they are a teenager living with Mom they might mean anything as they just don't know unless someone has told them something.

For most people it means two 4-barrel carbs, AKA dual quads. For Southern Ontarians, it might mean a case of beer hidden under the hood (a two-four, eh).

An 8 cylinder engine aka a V8