Personally I would just keep the 3900X you have especially if you plan to upgrade to the 4000 series later. We really have no idea of when the XT will release or what the price point will be. I have not even seen an actual AMD confirmation that the XT is actually coming out. Nor have we seen any tests. Sure the single core may actually hit higher but with some tweaks the 3900X can easily hit 4.650Ghz and all core 4.25-4.275 Ghz without an all core OC.
MSI MEG X570 Ace
AMD Ryzen 9 3900x
Corsair H115i Platinum, Corsair Crystal 570x RGB Case
32GB G-Skill Trident Z Neo 3600 [email protected]
MSI Geforce RTX 2080 Super Ventus SX OC w/ Acer Predator X34P