A Sony Authorized Dealer has confirmed that the WH-1000XM4 will retail for US$349.99, whenever Sony gets around to announcing its next-generation headphones. The WH-1000XM4 will be sold in black and silver too, just like the WH-1000XM3.
https://www.notebookcheck.net/Sony-Authorized-Dealer-leaks-US-349-99-price-tag-and-two-colour-options-for-Sony-WH-1000XM4.469635.0.html