Microsoft has released basically no official technical information about what exactly DirectStorage will be. However, extrapolating from what they have said, what would make the most sense would be for DirectStorage to be almost purely a software technology, with no hardware dependency other than possibly requiring NVMe. Some implementations of the DirectStorage APIs (eg. NVIDIA's RTX IO) may make use of other hardware features such as P2P DMA to make certain DirectStorage operations faster, and some use cases for DirectStorage may require certain GPU features (eg. for Sampler Feedback, or for decompressing assets on the GPU instead of on the CPU). But there's nothing to indicate that PCIe 4.0 will be a requirement in any way, and there's nothing that PCIe 4.0 offers in the way of new features that would be relevant; it's just a speed boost, and a PCIe 3.0 x8 SSD can be just as fast as a PCIe 4.0 x4 SSD.