If you go to anyone they will say you can have 7mb........... but your actual speed 3.9mb is unlikely to change who ever you are with. Every one tells us we can have 24mb............ but have never had more than 8mb.
Its all to do with contention on your lines. If it was just you then you would get the maximum, every neighbour who has ADSL is "sharing your bandwidth".... so the more neighbours... the less you get.
Some days here its fast, then after 6pm when everyone gets home the internet runs like a dog..... thats contention