The counterweights have nothing to do with the pistons and rods. A crank is balanced without them on - the counterweights counterbalance the rod throws, and nothing else. The pistons all counterbalance each other - they all need to be the same weight. They all go up and down in pairs, for every movement up or down or sideways (rod big ends) there is a corrsponding part going the opposite way somewhere else. A really good balancing job would alwo weigh each end of the rod, to make sure the mass of the big end that goes sideways back and forth is sonsistent as well.

Of course, all those forces aren't quite lined up (the two pistons going up are not in the same location fore/aft as the pistons going down) so a 4 cylinder will always be a little buzzy, regardless of how well are the parts are balanced.
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I'm JohnMc, and I approved this message.