ADSL EXREMELY SLOW

Since our ADSL line (4mbps line - uncapped) has been installed we have had endless problems with the speed of the ADSL Line, we just assumed that it was slow. As soon as we connect more than 4 devices to the network then everyone is unable to connect ot he Inernet or do anything. I then decided to change ISP's only to find out that our download speed is 512 kbps! When it should be over 3500! I tried to contact the call centre but this was all in vain as I was on hold for 40 minutes and could not get through. <br> This is just ridiculous as we are paying for a 4mbps uncapped line but we can't use it!!! Please can someone sort this out ASAP!!!!!