I agree with Roskoe. Good luck even spotting a deer at 400. I also agree that it's doable for a skilled marksman. The key word being skilled. I mean absolutely no offense, but none of us could consider ourselves skilled at 400 if we've only shot at 50. Make sense? There are many, many variables to consider at that distance. Wind being a major one for that round. Also, are you shooting uphill or downhill? A slight grade can make a huge difference over long distance shots. Also at 400, this might not be that huge of a difference, but humidity can affect a shot as well. If you sight in at 10% humidity, and take another shot at say 70%, your shot can be affected by as much as an inch at that range. Considering these things, you can see why people would call it irresponsible to shoot at an animal without being skilled at that distance. I'm fairly adept at 500 yard shots on paper under ideal conditions, but I wouldn't attempt that shot in the field with an animal. I get 12 or 13 inch groups from a kneeling position at 500. This group is nowhere near tight enough for a kill zone shot on an animal.

Also, consider this, if you did score a hit on a deer at 400, would you be confident that you would be able to re-aquire target fast enough to see how the animal jumped? This would tell you a lot about where you hit it and how to track it.

My suggestion? Find a place you can practice that range for a long time before ever attempting it in the field. In the mean time, work on the skill of getting close to the target. Personally, I get more satisfaction out of getting close undetected than I do out of a long range shot.

Good luck either way. Long range shooting can be quite rewarding.