Like you, I questioned what tactics I should use in my first couple of marathons. My club coach gave me sound advice which I've stuck to ever since.
From the start pace yourself consistently for the sub 4hr. Like others have said your 1/2 pb suggests this is a very realistic target.
If through 13m you're feeling good - stick with your pace If at 15m you're feeling good/strong - stick with the planned pace Same for the 18m mark - stick with the pace At 20m the race begins, you're feeling good so have a little think - Can I speed up? At 22m if you're still feeling good - Go for it. Get home as quick as possible.
In my last 3 marathons I've run 2 negative splits doing this and both times I've felt as though I had more in the tank at the end - Trust me this is one of the BEST feelings in the world (which inevitably makes you sign up for another marathon the following year as you're sure you can beat last years time). I've also found that this tactic breaks up the miles between 13-20 into bite size chunks.