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.