Close keyboard shortcut

I think it’s because Ctrl+F4, like Alt+F4 and Ctrl-Tab, is a system-level shortcut that works in all apps including the Windows Explorer desktop, and therefore Photoshop does not have control over that shortcut so it doesn’t list it.

There is a very long list of other similar standard Windows keyboard shortcuts that don’t appear in application-specific keyboard shortcut editors including non-Adobe apps. Those Windows shortcuts are defined at the system level. 

At the risk of repeating myself to change a Discussion to an Idea:

Why is CTRL F4 not listed as a keyboard shortcut in Photoshop?

It closes the current document and if you're not aware of what it does, it can be let's just say a very sweary moment.

Answers on a postcard.

And I've already added a script to do nothing to over-ride it, just so it won't happen again.

Why is CTRL F4 not down as Close in the list of keyboard shortcuts ? or preferences?

By @Ghoul Fool

Only Adobe knows for sure, the rest of us would be making guesses. My guess is that some keyboards don't have function keys and for those that do (most of them), the function keys have to be set on purpose to be Standard Function Keys. I use both Mac and Windows and on Day 1 of a new installation, I set the function keys to be Standard. Control/Command+W can be used by all, whereas Control + F4 is a Windows shortcut that has existed since at least Windows 2.0.

I followed your link and would expect to see this shortcut under the heading "Use Function Keys", but it's not there.

https://helpx.adobe.com/uk/photoshop/using/default-keyboard-shortcuts.html 

You might post an Idea instead of Discussion and ask for it to be included. Adobe staff reads those even when they don't reply.

Jane