Unlike V8 crankshafts 4-cylinder cranks do not have bob-weights attached when being balanced - the crank is just dynamically balanced bare. So this means that the counter-weights can be drastically lightened ... as long as it is done evenly on the up and the down counter weights.
A bit more engine rocking couples ensue ... but, Hey! ... it is not a smooth street cruiser you are driving.
Here is a good engine building web site that has excellent pics of a drastically knife-edged crank - which broke due to a machined stress riser rather than the actual lightening ....
Here:
Engine Building
Lots to learn from this site! The counter-weights have been reshaped on a lathes by the look of it.
HTH