You can pre-order now! https://smile.amazon.com/Expanse-Season-Four-Blu-ray/dp/B08KP1VV1D
There was a release for the UK market, not sure about the rest of the world: https://www.amazon.co.uk/The-Expanse-Season-Four-Blu-ray/dp/B08KP1VV1D
Dec. 15th (pre-order available).
Looks like season 4 is finally going to get a Blu-Ray release: https://www.amazon.com/dp/B08KP1VV1D/ref=cm_sw_r_cp_apa_i_F68FFbS04Q591
Release date Dec 15. But the price is ridiculous, $45.