How far from Exchange?
How many people in the area are using the internet at the same time as you? (contention rate)
Have you spoken to others in your area and found out what speed they are getting?
Have you tried it at say 03:00 in the morning? (when it is quiet, BB traffic wise)
Is your exchange equiped for 4Meg?
Some of the things you need to consider.
I am on TalkTalk and I have a package that is sold as being up to 8mb but I only get 0.9mb as the wxchange will not be un-bundled untill end of May 2008.