Okay...I think I understand a little better now. Sorry to be a nag but it would be really helpful if you could fill in your timeline -- it helps us to help you better!
Your file has been forwarded from the NVC to the Embassy -- has your husband received what is known here as Packet 3? This is a letter from the Embassy and not the NVC which states that processing has begun, and includes (amongst other things) a copy of DS-230, info on what documents need to be gathered, and how to schedule a medical, as well as the checklist I mentioned. If he has not received this yet, he is not "in the system" at London yet. As I said earlier, 4 weeks is on the long side but not unheard of.
I didn't go through the NVC as I filed directly through London doing a DCF, so I'm not fully conversant on what happens between the NVC and the Embassy -- has he received a case number yet? It should start LND2007... If you have this, you can call the Dept of State number I mentioned above and ask what his status is -- Packet 3 has been posted, or still waiting to be posted, etc etc. It is a US number and you will only pay what a regular old US long-distance phone call to DC would cost. The Embassy numbers do not have a back door -- sorry. They are known as the Extortion Line over in the UK Forum for a good reason! Generally, the Dept of State can help you out as they have the same info as the Embassy (most of the time). If they don't, they will tell you to call the Embassy. But you MUST have the case number.
I know how frustrating all of this can be -- if it's any comfort, you are not alone. Give your man a call or drop him an email today and find out if he has received anything from the Embassy directly, and if he has posted anything back to them. If he has, then we can give you some more advice. But this is the first step you have to do.
Best wishes from London!