Any DB string should do for CR4Ms as long as they have a steel core. Without that they won't work with the mag pups. Helicore Hybrids, Helicore Orchestral, Pirastro Flexocors, and Pirastro Obligatos seem to have been used a lot.
EDIT: Strike the Obligatos unless you don't want to use the mag pups. They have a synthetic core.
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