After being test-released in a small number of markets (e.g. France, Brazil and Singapore), the Android 9.0 Pie OS is now globally released to all the Galaxy Tab S4 devices. The update is already available in Poland, Slovakia, Switzerland, Italy, Kazahstan, Portugal, Romania & many more places worldwide. If it has not yet reached your place, it sooner or later will.
So, what is new in the 9.0 update? a bunch of things, actually: One UI interface, system-wide Night mode, HEIF images support, improvements for Bixby, Adaptive Battery optimization tool, improved notification management and all the usual Android 9.0 goodies. As always, you can download it manually by going into Settings, Software Update and selecting the Download updates manually option.