Salesforce by default displays numbers in a locale-specific manner - so in your locale, it's probably N,NNN etc. There is no way to override the display formatting.
What you could do is have your formula field return a text value like your other year fields, which should then return "NNNN" - it will display correctly in the UI, but you would need to convert it to a numeric number value again if you needed to use it elsewhere (or, you could keep the number formula for use in other formulas and create a different formula field for just the display value).
For the first option, you'll need to wrap your values in the
TEXT() function. Something like:
ISBLANK( End_Year__c ),
TEXT( YEAR( TODAY() ) ),
TEXT( VALUE( End_Year__c ) - VALUE( Start_Year__c ) )
A couple of other notes:
ISNULL() is deprecated -- use
- You don't need to wrap
VALUE() when you check if it's blank
- You didn't need to wrap
TEXT() and then
YEAR() already returns a numeric. But since this example returns a text field we still need that part.