1. Get the text, using scanf or so.
2. Trim it to get the last four chars.
3. use an index of the char array to check char by char (I think the function you need is IsDigit()).
4. if IsDigit is TRUE then you can continue...
5. at the end or at the beginning (probably it would be better) you can check if the value is equal or different than 0000.
I would use a while loop like:
bool bAllOk = checkstringDifferentThan('0000');
int iCurrentNumber = 1;
while bAllOk && iCurrentNumber < 5
bAllOk = IsDigit(plateNumbersArray[iCurrentNumber]);
You've just seen pseudocode... only to give you a rough idea on how to do it.
It is a nice program to do.
Divide and conquer! ;)
Updates due to your questions:
In order to check than a string is different than something: http://www.cplusplus.com/reference/string/string/compare/
] or http://www.cplusplus.com/reference/clibrary/cstring/strcmp/