One option:

if ($string =~ m!(^[a-z0-9]{4})$!) {
    $string =~ tr/[a-z]/[A-Z]/;
    # do something
} else  {
    # do some error
}
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This looks for the start of a string, exactly four characters from the class between lowercase a and lowercase z or between 0 and 9 inclusive, storing them, and then the end of the string.

If found, it uppercases it. (I use tr/// because it's faster than a substitution, and because leading digits may cause a warning about modifying a constant. uc is another option.)