If there are always at least five characters in the string then

/\D\d{4}$/
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if an acceptable string could be exactly four digits then

/
(?:     # either
    ^   # start of string
|       # or
    \D  # a non digit
)
\d{4}   # exactly four digits
$       # end of string
/x
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Update: /(?<!\d)\d{4}$/ covers all cases, I tend to forget about look behind because I have to use fairly ancient perls as well as the more modern varients and so code for the lowest denominator.