Yeah, I'm looking forward to getting one (at least) of these to play with.
I owned and used a few G3258, which some of you know, was a 3.2Ghz Haswell Pentium, with an unlocked multiplier. It was also know as the "Pentium Anniversary Edition", and came in a box with a "20th Anniversary of Pentium" in a yellow banner across the bottom.
Anyways, mobo mfg's soon learned how to allow OCing those CPUs, not on a hugely-expensive "Z" mobo, but on an entry-level H81 board. They would easily reach 4.0Ghz in nearly any board that supported multiplier changes, and sometimes 4.2-4.3Ghz or higher.
So, that was the original advent of the 4Ghz Pentium, albeit overclocked. Unfortunately, it was only 2C/2T, and some AAA video games even refused to launch, on a CPU with less than 4 threads. So they became DOA for gaming.
Similar to those, the 2C/2T Skylake Pentium G4400 CPUs, could be BCLK OCed in certain (ASRock) Z170 mobos, I got mine above 4.0Ghz as well. But you couldn't use the iGPU when BCLK OCing, and they suffered from the only 2C/2T problem too.
Kaby Lake introduced the first 2C/4T Pentium, the G4560, but that was only 3.5Ghz stock, no turbo or unlocked multi, and no way (that I know of) to BCLK OC them.
Therefore, this recent release of the G6400, is basically the culmination of all of those:
Stock 4.0Ghz clock, and 2C/4T configuration. The only thing missing is AVX opcodes, which could still be a problem for gaming purposes.