At 4 Km, with a signal level between -60dBm / -65dBm and a Noise floor arround -90dbm your link must work OK
Using MCS15, with a 20Mhz Channel width, you get 130mbps, so for your 20mbps pipe it must perform OK.
I have done some tests using MCS10 at 20Mhz channel width (39mbps), with Nanobridge, and it can pass 20mps without problem.
http://dl.ubnt.com/UBNT-MIMO_Data_Rates_2.pdf