Forgot password on Samsung Galaxy Tab S4 - how to reset?
If you have time, let the battery drain. Then power up the tablet with the charger. Press Power + Volume Up to go to recovery (until you see the recovery screen) and reset the tablet.
If not, hold Power + Volume Down for up to 45 seconds to restart the tablet. When it restarts, press Power + Volume Up to go to recovery (until you see the recovery screen) and reset the tablet.


Sources: 

Samsung Galaxy Tab S4 Reset Guide: How to Master Reset, Reset Network Settings, Soft Reset your Tablet

Samsung Galaxy Tab S4 - Factory Data Reset (Powered Off)