I think what you are looking for is:

if( $test =~ /^\d{4}$/ ) {
    ...
}
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This will match:
'^': anchor start of string
\d{4}: exactly 4 digits
$: anchor end of string

if you know it is always going to be digits, then length would probably be eaiser:

if( length($test) == 4 ) {
    ...
}
[download]