[Samsung Galaxy Tab S4 Guides, News, & Discussion] m0byn:
For the case that others might have the same questions. I managed to get LineageOS 17.1 and LineageOS 20 up an running. How did I do it?

Install TWRP WIFI version from official TWRP website (also works on LTE version)

Format data -> type "yes" and cofirm

Wipe Cache, Dalvik, System

Flash LineageOS 17.1 arm64_bvS from AndyYan Sourceforge repository (if you start from Android 10, which you should because you should obviously install the latest available Samsung stock firmware)

Flash DM-Verity disabler (I installed 11.02.2020 version) to disable DM-Verity and also encryption

Voilà! It should run! If you want to run LineageOS 20 proceed with

Reboot to TWRP

Flash LineageOS 20 arm64_b versions (I installed bvN)

Flash DM-Verity disabler again (not sure if it has to be flashed again, I do it)

Voilà!

This is basically the only way it worked. I tried many others such as:

Different versions of treble roms

Flash LineageOS 20 straight away not flashing 17.1 before

Trying to keep encryption enabled with DM-Verity disabler

Trying to flash a patched boot.img instead of DM-Verity disabler to keep encryption enabled

but nothing worked.

I suspect encryption is not working since the tablet uses FDE instead of FBE...but I do not have enough knowledge to address such an issue.

Important: screen lock by gesture, pin, or password does not work!!

EDIT: after some days of usage it appears to be unstable. Maybe try not to skip any major LineageOS update, so first install 17.1, then 18.1 and so on until you arrive at the desired one. I did not test this approach.

I tried to figure out any way to enable encryption, but did not succeed. The ways I tried:

Install LineageOS, then flash no-verity-opt-encrypt.zip: results in known ERROR 1 when installing no-verity-opt-encrypt.zip. Just google the error for clarification.

Flash an old TWRP (3.3) in order to not run into ERROR 1 to install no-verity-opt-encrypt.zip. Installed, but did not find fstab.qcom file, so worthless.

Install DM-Verity disabler renamed to Disable_Dm-Verity_enfec_11.02.2020.zip not to disable encryption. Can boot but tablet is not encrypted and also pin lock does not work.

Flash a patched boot.img, install LineageOS 17.1, mount vendor partition in TWRP and edit vendor/etc/fstab.qcom manually via adb pull and adb push.

Changed forceencrypt=footer to fileencryption=ice did not work

Changed forceencrypt=footer to encryptable=footer which boots up. Also, one can set a pin lock, but then when entering it again after setting it, it will not be recognized anymore. Moreover, when trying to encrypt the tablet it just crashes and reboots.

Finally, I think there does not exist a way to flash LineageOS AND enable encryption on this device. I suspect the cause is that the Tab S4 uses FDE, and not FBE. I am sure there exists some way to make it work. I am not skilled enough for that though. One last point, obviously I searched for the setting in Developer Options to convert encryption from FDE to FBE, but sadly this setting does not exist (not in stock, nor in LineageOS).

Conclusion: I reverted back to stock since I have to have encryption enabled. If this is not a requirement for you then try the above mentioned steps. Also, try not skipping a major LineageOS update, maybe this fixes the instability issues! Also: REPORT your experiences in this thread!