AnandTech wrote a detailed article about Toshiba's PCI Express 4.0 solid state disk plans. The main gist is that the company is primarily targeting datacenters, at least for now the company isn't giving a lot of priority to the consumer segment. Toshiba predicts PCI Express 4.0 will remain a niche enthusiast feature for a while longer, which is logical as it's currently only available for systems with AMD's X570 chipset.
Even though there are a few PCIe Gen4 SSDs already available in the consumer segment, Toshiba is predicting that PCIe Gen4 will remain a niche enthusiast feature for a while longer. Since Toshiba's client SSD sales are almost exclusively to OEMs rather than direct to consumers in the form of retail products, they aren't under much pressure yet to switch these products over to PCIe Gen4 and haven't made any official statements about upcoming products for this segment. We probably won't see a successor to the XG6 and XG6-P until Toshiba's 1xx-layer 3D NAND is ready.