By default, the 4C Professional offers six-key rollover, but you can toggle on NKRO with Fn+F11. The NKRO is enabled via the USB cable and does not require a PS/2 adapter. The 6KRO worked just fine in our tests, and using the Microsoft Applied Science and Aqua Key tests, we confirmed that NKRO was excellent as well. We were unable to find any flaws.
I recorded myself typing on the keyboard; playing with the WASD keys, directional keys, and space bar to simulate gaming; and clacked a few keys individually to get a simple, clear example of the sounds you'll hear. To capture and evaluate the nature of the sound, I used a condenser mic positioned above the keyboard (approximately where my head would normally be), where the projected sound was most likely to be identical to what reached my ears.
One thing I noticed right away is that there’s an extra high-pitched metallic “ping.” I’ve noticed a similar effect with Corsair's Vengeance K90. And it's curious, because those two keyboards have very different backplate constructions. The 4C Professional is more of a bowl design, with that aforementioned foam underneath the top plate, whereas the K90’s switches are perched right on top of the backplate with no bowl whatsoever. So it goes.
The 4C Professional has a rather “active” sound. I can live with it (and have for weeks), but some users may find the constant “ting” aggravating.