Off late, I could not find any 4 spatial stream capable Access Points in Cisco. Has been going through the datasheets of flagship models in Outdoors (1572EAC) and Indoor (3800E). These are 4 Rx & 4 Tx but with 3 spatial streams.
What is the 4th Tx Rx Antenna in 4*4 is used for in these AP models as its not been for facilitating the 4th spatial stream?
Not from the classical AP-line, but at least from Cisco and 4-stream: https://meraki.cisco.com/products/wireless/mr53
To my understanding (hopefully the Wireless Pros will correct or add to that), it's used for beamforming as it needs one more antenna then spatial streams.