The winding is typically metal (different metals are used), not fluorocarbon, nylon or similar composite. I've never worried about the core—the composition of the metal winding is more important.
Aquila used to make a red baritone single for use as a low 4th string, but last I heard, they'd discontinued that, prior to their reformulation of the reds, and I don't know if they've started selling them again. Their red strings have a higher unit density (due to a copper infusion), which makes them much more suitable as low strings—with a thinner diameter as well (no need to alter the standard nut notch). The Aquila red was the only non-wound string I could recommend for a bari low 4th. At the time, the red 4th was only available as a single; curiously, the red sets included wound strings for the 3rd and 4th strings.
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