For me and my one marathons worth of experience 3-4 x 20+ mile runs are a definite must. No way I could have got around LN after only 2 x 20mile runs. There were some tough hills in it. That one at mile 21 will never be forgotten.
Also mentioned is the specific training, think that is a good point. So try to emulate the course terrain in your LSR.
Just googled Larmer tree marathon, wouldn't expect sub4 in that " It will encompass some of the most challenging parts of the three counties."
Good luck sure the sub4 will happen soon on the right course.
I'm no expert but all my long runs were tough and like you a few weeks before, I had thought I might just be able to hold on for a 4 hour finish. But I stuck to plan A, a 3:45 finish and paced for that, apart from dying on a big hill at mile 22 after many many other big hills.