Just to be pedantic DDR4-4266 is non-standard and so won't be found in any mainstream OEM's laptops. LPDDR4X-4266, soldered to the board instead of socketed and with a lower voltage, is indeed an official thing though.
Right, JEDEC standards for DDR4 only go up to 3200. But 4266 is within the range of overclocking on desktop systems and only a few percent faster than the fastest SODIMMs on the market, so it's at least somewhat useful as a point of comparison.
LPDDR memories are developed with more of a focus on per-pin bandwidth than standard DDR because mobile devices are more constrained on pin count and power. But Apple's now shipping an LPDDR interface that's just as wide as the standard for desktops, and reaping the benefits of the extra bandwidth.
