Microsoft sends out invites for a 6 October 2015 event involving "exciting news about Windows 10 devices"-- meaning the company will (probably) launch a new Surface tablet and a couple of Windows 10 Mobile devices.
A number of news sources claim to know what Microsoft has in store for the event. For instance The Verge says the company will not only launch the Surface Pro 4, but also the Lumia 950/950 XL smartphones and even a Band wearable sequel.
Reportedly the Surface Pro 4 retains the same form factor and charging mechanism as the Pro 3, but might include features such as Windows Hello support and a higher display resolution. Other rumours suggest the possibility of a larger, 14-inch version of the device.