Adsl Line - never running at the speed we are paying for!!

We have had our 4mbps adsl line since June this year - AND it has never run at this speed - always just over 2mbps. Telkom has come out to our premises so many times, and still can't get it right - Maybe if they stop sending the "cable" guys out to us and sending the correct people out to us, we MIGHT get somewhere. This is total ***** - and you never get a credit either. What does it take??? or must I can cancel my line??