I have never seen a USB adapter that claims 4x4 on 802.11ac. The closest I have seen is the asus usb-ac68.
It runs 4x4 on 2.4 and 3x3 on 5g 802.11ac.
I am not so sure how well 4x4 mimo really works, the more overlapping signals the less chance it has to work.
Your best bet if you want the speed is to wait maybe 6 months until more 802.11ax equipment is on the market. It is all suppose to finalized by the end of the year....assuming they do not delay it again.