The firmware changelog is dated September 14, but the Surface Update History page is listing the new improvement just now. Here are all the changes for this firmware update:

Surface Fingerprint Sensor – Biometric device: 2.2.11.0 Enables support for Surface Pro Type Cover and the Surface Pro Signature Type Covers. Surface PTP Filter – HID: 1.0.173.1 Enables support for Surface Pro Type Cover and the Surface Pro Signature Type Covers. Surface Pro Type Cover Integration – HID: 1.0.2.0 Enables support for Surface Pro Type Cover and the Surface Pro Signature Type Covers. Surface Smart Cover Backlight – System device: 3.0.0.0 Enables support for Surface Pro Type Cover and the Surface Pro Signature Type Covers.

The new September 2017 firmware update for Surface Pro 4 should download and install automatically. However, if you have not received the update on your Surface, then go to Settings > Update & security > Windows Update and try to force the download manually.

Surface Pro 4 | See at Microsoft Surface Pro Type Cover | See at Microsoft

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