My advice, forget sub 4hr. It's not that you can't do it, or won't do it (although it is very very unlikely), but that having that in mind will disrupt a very achievable goal of enjoying the completion of your first marathon. (It is your first, right?) Off your present HM PB I'd think you'd be looking at 4:30 for the Full, so that's the kind of figure you have to get into your head and spend time on your feet preparing for.
There's a huge amount to learn from your first marathon (from every marathon really) and what you learn will tell you what you need for subsequent ones in which your goals can be more time oriented. Right now you need to gradually get used to longer and longer runs, to work your aerobic systems, and you only really have three months before you've got to be up to your longest training distance. So, for this one, run lots (gradually building up) and run slow (with a little speed lightly sprinkled in).