You can’t change the default format. It is hardcoded to the locale of the system and all of them use a 2-digit year by default. The reason is that there are simply too many default settings to allow even a modest percentage of them to be displayed via dialogs. This question is related.
The only workaround is to:
- Create a cell style that uses the required display format.
- Save this empty sheet as a template.
- Set this template to be the default.
You can then manual apply the required cell style to each cell containing a date as required. Bugs fdo#30821 (Sorry, Writer table context rather than Calc) and fdo#41044 are related.