I'm fairly sure you can do it all through Ticketmaster, but my friend I go with sorted it all out so I'm not 100% sure.
Edit- He reckons he had to join NFL UK, so it's probably best to do that just to be sure. He also isn't sure if you can do just Wembley, maybe it was only offered because we had just Wembley previously and they can't exactly force us to go to a new venue. The website certainly suggests the tickets are for "all 4 games" but I do know the Twickenham deal is technically up so we may not have to go there again. It was for 3 games over 3 years but they did state it could be "1 game this year, then 2 games next year and then 0 games in year 3" which we are currently on for. Reports are they want to extend it, I'm hoping not.