If you already have the Hub 4, when you upgrade to the 1gig service it's only necessary for someone in VM to press a few buttons. So there's no incentive for them to go to the trouble and expense of sending you a new Hub.
It's unfortunate that your Hub 4 has a noisy fan. My Hub 4 is in a bedroom and it's virtually silent. It might be worth trying for a replacement Hub 4 on the basis that your present one is faulty, rather than paying extra for a faster connection that you don't need. Unless you have a large family of extremely heavy users you won't notice any difference.
Bear in mind that any upgrade price offer is a special deal to get you to sign up. After 18 months you may get an unpleasant surprise at the normal price.