Sorry to break it to you but you can’t score 400/400 in JAMB. It hasn’t been done in the history of JAMB in Nigeria.
You are not a robot with 100% accuracy, no matter how intelligent you are. You will make mistakes.
If you want to smash JAMB, aiming for 400 isn’t bad but try to set a feasible goal like 320, 350, or even 370.
No institution will reject you or change your course if you scored above 300 in JAMB.
How Does JAMB set their questions
The questions are set by those who work at JAMB, they are mostly experienced teachers, lecturers, and professors.
They set the questions for different topics in the UTME syllabus. There’s no copy and paste work, new questions are created every year.