How can I press "Tab" to enter 4 spaces

Press the “Tab” key to lose focus, not to indent. I want to press the “Tab” key to enter 4 spaces instead of leaving the editor.

The short answer is you shouldn’t do that, because it will make your page seriously unusable for a lot of people. See for example the Web Content Accessibility Guidelines.

The long answer is that if you really don’t care about users who don’t use a pointing device (which includes all blind users but also quite a lot of people who for other reasons are limited to keyboard input), you can define a custom key binding (see the keymap helper) for tab just like you can for all other keys, and bind it to a function that inserts a tab.