Telkom downgraded my line without my consent!

Have had a 4Mbps line with Telkom for about two years no with no problem. Suddenly on 28 September the line was 2Mbps. I did not ask for this or nothing, and now they are unable to put me back on 4Mbps cause my exchange does not support it. Is this the way it is now? Slower speed in stead of faster speeds? I need the 4Mbps line, it is my livelihood... Please assist! Anyone?...