$re = '/(?:\G(?!^)|^((?:/[^/\n]*){3}/))([^\d\n]*)\d/m';
$str = '/path/to/something/1234/end
/path/to/something/12/1234/end
/FWS/teis-facade-rest_v712/flk/tariffs/114776/14/options
/mem-utms-facade/v1.2/rest/URA/ST00006486/note
/FWS/spg-facade/eis/2650114465322000006/contract
/FWS/fanotical-soyarty-service/flk/lk_2456882/bonusFlk';
$subst = "$1$2#";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php