[Thinkpad Tablet General] matshias:
WARNING!!! Be careful what you do here. One mistake and the device is soft-bricked. I take no responsibility for bricked devices, lost warranty or even OTAs not working!! Booting and/or flashing files from this post is on your own risk.

Requirements

Unlocked bootloader

Lenovo Tab 4 Plus with and without LTE and both 8 and 10 inch versions (TB-X704L/F, TB-8704X/F) are supported

Known Issues

WARNING! Our device uses dm-verity (verified boot). The system partition should remain read-only. Otherwise a bootloop will occur. You can flash SuperSU to resolve this but OTA's are no longer possible! You have to restore a factory image or clean system backup to receive OTAs

Download

TWRP for TB-8704X/F (8 inch plus): AndroidFileHost

TWRP for TB-8704V (8 inch plus Verizon): AndroidFileHost (provided by [MENTION=2185657]3m1k[/MENTION])

TWRP for TB-X704L/F (10.1 inch plus): AndroidFileHost

TWRP for TB-8504X/F (8 inch): AndroidFileHost (touch input may only work after connecting mouse)

TWRP for TB-X304L/F (10.1 inch): AndroidFileHost

Instructions

Install fastboot and adb on your PC, e.g. from here

Enable developer options and in there select to unlock your bootloader by enabling OEM unlock and enable USB debugging

Connect your PC to your tablet and run

adb reboot bootloader

using adb on command line. The tablet will reboot into bootloader mode where you will only see the Lenovo logo. Now you use

fastboot oem unlock-go

to unlock. This will factory reset your device

Setup the tablet again and reboot again to the bootloader

Then run

fastboot boot twrp-3.2.3-0-<your-filename>.img

to temporarily boot into TWRP. You can also flash if you are sure ;)

Select to keep system read only when TWRP starts to avoid modification which will make OTAs impossible

Backup at least system and boot partitions. Choose "System Image" for the system partition backup. Keep this backup for OTA updates.

Optionally flash SuperSU or Magisk in TWRP which should install system less. Keep OEM unlock enabled if you flash or modify anything

Writable System Partition

Some mods and root apps require write access to the system partition. By default the system partition is read-only to prevent any modification. Please note that if your system partition is modified in any way it needs to be restored to its original state before a system update (OTA) can be applied.

[MENTION=6612046]dywersant[/MENTION] has created a Magisk module here to achieve write access

[MENTION=3190122]KreAch3R[/MENTION] has updated the Magisk module to the latest version here

System Updates (OTA)

In order to successfully apply Lenovo system updates after root you have to restore the original boot, recovery (if you flashed TWRP) and system partitions from your current installed version (e.g. S000037) for your variant of the tablet. This will NOT delete your apps and data but will unroot. Note that OTAs are block based and always check the contents of most of the partitions. This means that all of these partitions (except your data) have to be original and have to be on the exact same version (e.g. S000037) for the update to succeed.

Credits

[MENTION=6618747]wonderfulme[/MENTION] - for testing the 8" version

[MENTION=1487299]Teddy01[/MENTION] - for testing the 10" version

[MENTION=2185657]3m1k[/MENTION] - for providing the 8" Verizon version

TeamWin

Source

TWRP Manifest: https://github.com/minimal-manifest-twrp/platform_manifest_twrp_omni/tree/twrp-7.1

Device Tree (TB-8704X): https://github.com/Matshias/twrp_android_device_tb_8704x/tree/android-7.1

Device Tree (TB-8704V): https://github.com/3m1k/twrp_android_device_tb_8704v

Device Tree (TB-X704L): https://github.com/Matshias/twrp_android_device_tb_x704l/tree/android-7.1

Device Tree (TB-8504X): https://github.com/Matshias/twrp_android_device_tb_8504x/tree/android-7.1

Device Tree (TB-X304L): https://github.com/Matshias/twrp_android_device_tb_x304l/tree/android-7.1

Changelog

v3:

Update to TWRP 3.2.3-0

v2:

Update to TWRP 3.2.1-0

v1:

Initial Release