According to the official spec sheet -- obtained by 9to5Google -- we're looking at a Snapdragon 855 processor, 6GB of RAM and up to a 90Hz display. Marked differences between the Pixel 4 and Pixel 4 XL are inevitably the screen size (5.7-inch vs 6.3-inch) and battery (2,800 mAh vs 3,700 mAh). Both offer a choice of 64GB or 128GB storage.
Under the processor information, both devices are labeled as having a "Pixel Neural Core," which appears to be the new name for the "Pixel Visual Core," as found in the Pixel 3. This is the imaging chip responsible for the Pixel's excellent low-light camera, so we can probably expect even greater imaging abilities with the new handsets.
Interestingly, there appears to be changes to what comes in the box. According to 9to5Google, the device will come with a 1m USB-C to USB-C cable, an 18W USB-C power adapter, a Quick Switch Adapter (female USB-A to male USB-C), a SIM tool and a Quick Start guide. This means it won't come with the USB-C headphone adaptor or USB-C earbuds that the Pixel 3 shipped with -- although this might only be the case in the US, as some countries legally require phone manufacturers to include headphones.
There's very little left to know about the Pixel 4 now, and it seems that the official launch event on October 15th will act mainly as a demonstration of its new and improved features, rather than showcasing any big new reveal. But who knows? Maybe Google still has a few surprises up its sleeve.