It likely uses all of them at the same time to get its so called 600m and uses 3 of the 4 to get the 1300 on the 802.11ac.
The days of messing with the antenna on routers are long gone. All the fancy high speeds are dependent on overlapping signals and even the spacing and size of the antenna is critical. If you even did something simple like put large omni directional on all 4 the spacing between the antenna may not be optimal any more.
If you were to put direction antenna on you likely would drop your router to 150m maximum. The router in general is smart enough to use whatever antenna is strongest to each client.
I would use a direction bridge device instead to solve the issue. These are directional antenna with the bridge...