Samsung has announced that Linux on Dex is coming to more devices. Previously it was only available on non-LTE models of the Galaxy Tab S4 and on the Galaxy Note 9. Per an email that Samsung sent on Monday support is coming to the Android Pie builds of the S9, S9+, S10e, S10+, Tab S4, and Tab S5e.
Based on interaction with others (and also being my own personal story) there are owners of the TAB S4 that haven’t yet received Linux on Dex support that wait with anticipation for support to come. I’ve not been able to confirm compatibility yet as the Pie build of Android isn’t yet available for my device. The Linux on Dex page had previously stated that none of the LTE Tab S4 models were supported. The page now only states that the Verizon LTE tablets are not supported. I hope that this means that support for my device is coming. For now the only option is to wait.
Update (2019-April 30): Today I received the Android Pie update for the Galaxy Tab S4. It does indeed have support for Linux on Dex (finally!).