Asus makes quality routers but do not think that the biggest number is always the best, other models in their line may meet your needs better.
So to use things like 4x4 your end devices must also have 4 antenna which is rare. Most device have 2 antenna which caps the speed at 300m on 802.11n and 900m on 802.11ac. The MU-mimo feature also requires the end devices to support them. Then to use all three radio in the device you must manually balance your device across them.
Then again most this stuff deals with the speed the connection runs not the range. The transmit power is regulated by the government and even the cheapest asus router likely puts out the same radio power as the most expensive. This means more or less they have...