Paying for 4mbps ADSL line with 2mbps line speed

I have logged a call regarding the slow connection speed that I receive. Every second day my call status is changed to closed with some lame excuse and no improvement in service. This is not acceptable. I am paying for a 4mbps line speed and expect the line speed to be in the region of 4mbps and not where I am currently achieving 2,5mbps on average. I have seen speeds below 2mbps and every time I do a \port recreate"the speed drops even further. This is really poor service and to think that they were awarded the best ISP....."""