Take a look at perlre, you are using wrong the syntax!

  \d     ## means 1 digit.
  \d\d   ## means 2 digits.
  \d+    ## means 1 or more digits
  \d{2,} ## 2 or more
  \d{3}  ## means 3 digits.
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And what you are doing just doesn't exists as you think:

  \d+{4} ## means one or more digits and the text "{4}".
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Actually your REGEXP doesn't compile! I just don't know from where you got the idea of \d+ with {4} after it. And when you use it right you put a lot of \d before it: \d\d\d\d{4}. Note that "\d\d" already means 2 digits, and \d\d\d\d already means what you want (check if the string have only 4 digits) but you put {4} in the last, what means 7 digits!

See perlre, and don't forget that perldoc.com is your friend, so, you can call it every time.

Good luck.