Is that 2km with CLS (Clear Line of Sight) or Near CLS?

Actually, I am not "a competent wireless networking company" but I'm thinking if those sights are in Nairobi, then you'd better use other methods of inter-connecting the offices than wireless bridges. Nairobi's "air" is saturated with wireless signals, and I hear those cause interference and hence performance degradation, especially when devices happen to transmit on the same channels. Getting the offices connected to an ISP and then doing VPN tunnels between them should be much sleeker. You can use one ISP to give you local loops and given that you are connecting four sites gives you the advantage of bargaining hard with the ISP.