Concrete with metal in the way. Be glad you can have at least some signal. If you cannot make some poles on both sides to shoot over the building, try to find some retranslation point. If no go, try 2,4ghz instead 5ghz, use high gain dish antennas with side shields. You might be successful to shoot through. Even though you will need to use illegal power.
Sxt is panel antenna. You need to concentrate the beam into very narrow angle, especially if you try to bounce. 5g is better for bouncing, 2g for penetrating the walls.
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You would need all possible power, so do not reduce it.
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Try to use NV2 and not nstreme or plain 802.11. It works better in these situations.
You can try Dynadish/LHG/QRT with higher gain or SEXTANT for higher power and a little bit of gain.
You can do it! 
Make sure the frequency you choose it’s a clean channel by doing a Wireless Scan and a Spectral-Scan (/interface wireless spectral-scan wlan1) from both sides.
You need a very clean spectrum to make NLOS links.
4Mbps is a low speed so I’m sure you’ll achieve that without any retranslation point and only using higher gain antennas.
I suggest you to use Dynadish/QRT/SEXTANT and exclude SXT/LHG because of their lower power radio.