Aside from AMD's recently released X570 chipset with PCIe 4.0 support nothing is on the horizon.
Intel is not releasing anything substantial until 2021. They might opt to skip PCIe 4.0, but it is somewhat doubtful.
There are only a handful of storage devices that require PCIe 4.0.
In the enterprise sector, I wouldn't be surprised to see PCIe 4.0 network and storage controllers already on the market.
I suppose it really comes down to whether baking in the added bandwidth of PCIe 4.0 is effectively cheaper than widening the bus for storage devices. Basically requires faster switching and more power from the chipset. That may be counterproductive when it comes to laptop battery life. You can already get PCIe 3.0 storage cards that slot into x16 slots, that would be equivalent to PCIe 4.0 4x.
Honestly might have to wait for additional process node shrinks before it makes sense for mobile devices.