They'll probably need to do another credit check on you for having more than 3 lines. I assume the guy said 3 only either because he did one and it was declined for an additional, or to have more than 3 you need to be on a small business account of some degree.
I'd recommend calling up on 150 from a Virgin phone or 0345 454 1111 from another phone to discuss your options with the sales guys.
Thank you your reply and PM. Im sorry to say that after waiting here and twitter for a reply, at least you replied here, we have taken our business elsewhere.
Im at a complete loss though why your system will not allow someone 4 contract phones for one family. This was for a Christmas present so the soloution of actually writing a pen and paper letter to a modern media company to get a answer is crazy.