For what it’s worth, I did 4:02 in first marathon (2012) using one of the Higdon Novice plans. Had an 86-min 10 mile in the run-up but sub-4 was always going to be a stretch with the low mileage I was on - only around 25 miles a week. The plan had no speed or tempo, just the odd few miles at MP. A classic ‘get you around’ plan.
The following year, the meno plan was introduced as he was mentoring the 2012 graduates. I ran 3:47 in 2013 off that - a good mixture of speed, tempo, long, easy. And lots of advice about how to run them. I found it excellent, doing it at about 35-40 miles per week. Used it twice more, in fact, getting down to 3:29 by Cork 2015.
The key was not the day-to-day details of the plan itself, but rather its overall scope and structure: increased mileage, challenging sessions, long runs with added challenges (in and around MP, variable pace, etc). This all helps with learning to push yourself a little harder, run with discomfort, confidence - all those things you need to break through arbitrary barriers such as sub-4, sub-3:45, sub-3:30 etc. Some of it is mental, but the majority is just doing the work diligently, running at the right paces so your body makes the adaptations you need.
My old log at https://touch.boards.ie/thread/2057009350
documents that first sub-4 training and marathon, if it’s of any interest to anyone.
Best of luck to anyone attempting. I’ve no doubt that almost anyone can do it with a bit of discipline, and of course you have to make the start line - sometimes the hardest challenge of all, especially earlier in your running career.