Baseline (Widescreen) Solution & Issues
Ultra/Super-Wide (21:9/32:9) Specific Solution & Issues
The game needs a HEX edit.
Use any program for Hex editing such as HxD.
Navigate to your game files (Right click the game in steam > Properties > Local Files > Browse Local Files) In the top-level folder look for DevilMayCry4SpecialEdition.exe
Open it in your Hex editor.
Look for "39 8E E3 3F" and change it to "8E E3 18 40"
There should be only one entry to find.
Now click Save and close.
Launch the game
Note how the game may appear to be Windowed at 2560x1440, press alt+enter to fullscreen
Note that the menus are still stretched and look like s*%# (don't worry about this, I haven't found a way to modify the menus, but your in-game will be the right resolution)
Eyefinity / Surround Specific Solution & Issues
No support for Eyefinity / Surround - max Resolution in Game is 2560 x 1440 - max Aspect is 16:9
4k UHD Specific Solution & Issues