FR-4 sounds like the model number of a new truck, maybe a fighter jet. Instead, it is one of the most common components inside electronic equipment, the printed circuit board.
Certainly, there are many types of PCB materials, but the one found in nearly every product since the 1960s is a flame proof (that's where the FR comes from) woven fiberglass cloth with an epoxy resin binder that connects circuit components together.
You might think this ubiquitous carrier of the resistors, capacitors, semiconductors, and tubes is neutral to sound quality. But you would be wrong.
I can tell you from personal experience that a hand wired prototype circuit sounds different than the exact same components mounted onto FR-4.
Part of the reason we hear differences concerns wires vs. tiny flat copper traces found on PCBs. But more than that, the fiberglass materials themselves act like a capacitor, absorbing and distributing the electrical pulses that make our music.
But FR-4, while not perfect, is certainly one of the best choices we have if sane pricing is to be respected. Other, better, materials like Teflon can certainly help sound quality but the PCB itself can often cost as much (or more) as all the other costs combined.
It's good we have a reasonable means of connecting our components together.
