Cutting off Crank counterweights
The 4 cylinder's primary balance does not come from the counterweights. As soon as I saw what they were doing, I knew what they would find.
As George noted in post #3, the counterweights on a 4 cylinder engine are used to keep internal bending of the crankshaft, and the forces on the main bearings, under control.
I never thought about the inertia though.. that makes sense.