Running slowly is a skill you need to learn. Running long is hard in itself(hence wall hitting), so holding yourself back is a way to protect yourself. If youa re constantly hitting the wall, you are constantly breaking your body down, by the time you get to race day you will be exhausted and you won't hit your target of sub 4.
Run slowly, even if it feels unnatural. If you really want to go sub 4, then holding yourself back on a long run seems a small price to pay.
I have written schedules for clients who started at 4.45 and are now aiming for 3.50........so I know it works!