Seemingly everything regarding the Pixel 4 has already leaked, but there might still be room on the pile for more. Nikkei sources claim Google has started "test production" on a 5G version of the Pixel 4 in China (though it might be produced in Vietnam). It would still use the Snapdragon 855 expected in the LTE models, according to the tipsters. There's no mention of other functional changes, so don't count on getting more than faster wireless if this proves accurate.
Whether or not you'll hear about a 5G variant at Google's October 15th event is another matter. Test production suggests it's still early, and Nikkei's contacts said Google might wait to unveil a 5G version in spring alongside a new budget Pixel phone (the Pixel 4a?). Not that there's much of a rush to do so. Carriers' 5G networks are still hard to find, and existing cellular chipsets frequently demand compromises in battery life.
Google hasn't commented on the claimed leak. However, a spring 5G update might make sense beyond waiting for the technology to mature. By spring 2020, all of Google Fi's carrier partners (Sprint, T-Mobile and US Cellular) will have 5G running in some capacity. A 5G Pixel 4 would serve as a showcase for those upgrades, and might keep people subscribed to Fi where they'd otherwise be tempted to leave for other providers.