# I've changed all x-es to spaces (for comparison).
for my $line ('1234', ' 1234', '1234 ', ' 1234 ', 
			'12345', ' 12345 ',, ' 123456 '){
	print map { sprintf "%10s: $_\n", "'$line'" } join ' ', 
		map { $line =~ qr/$_/x ? 'OK' : 'NO' } 
			'(?<! \d) \d{4} (?! \d)', 
			'(?<! \D) \d{4} (?! \D)', 
			'\b       \d{4} \b     ', 
			'\B       \d{4} \B     ', 
	}
__END__
    '1234': OK OK OK NO
   ' 1234': OK NO OK NO
   '1234 ': OK NO OK NO
  ' 1234 ': OK NO OK NO
   '12345': NO OK NO NO
 ' 12345 ': NO NO NO NO
' 123456 ': NO OK NO OK