Per Wario 64, the Twitter demigod who picks up on sales and store listings as soon as they happen, Valkyria Chronicles 4 has appeared, but is not quite ready for pre-order sales. The game is live for the PlayStation, Xbox One, and Switch in North America. One more thing: it also has a release date.
According to this Amazon listing, all versions of Valkyria Chronicles 4 will be launching on October 16, 2018. Some confusion was caused initially, as the previously-announced Switch version listing did not go up at the same time as the other two. The Switch version of Valkyria Chronicles 4 will be releasing after the PlayStation 4 version in Japan, so there was concern that would also be the case here.
Valkyria Chronicles 4 is the first true sequel since the third game on the PSP, which was never localized in English. In-between has been a remaster of the original game for multiple platforms, along with an action-based spinoff that did not perform well in sales or review scores.
Source: Twitter