I have been in your situation and get it. Having said that, 400 yards is a long ways to shoot at an animal without a lot of practice. I hunt out west where we can get long shots and rarely have had to take a shot over 300 yards.
The problem with a 400 yard zero is that if you misjudge an animal at your mid-range trajectory, you will shoot over them.
I have used the maximum point blank range zero for most of my life and have yet to miss an animal because of hold over or possibly hold under with a 400 yard zero.
This could be helpful in finding your MPBZ:
http://www.shooterscalculator.com/point-blank-range.php
I would strongly advise against shooting at game at 300 + yards without some form of practice. If you don't have a range that allows for that distance, then pass on it for this year and come up with a plan for next year. If you still think that you are up to the task, I would spend as much time as possible looking at the game animal you are hunting between 200 to 400 yards. Often it is difficult to confirm whether or not it is legal to shoot, if that is a requirement, without additional, expensive equipment.
Best of luck,
Matt