Match all substrings that end with 4 digits using regular expressions

PHP uses PCRE-style regexes which let you do lookbehinds. You can use this to see if there are 4 digits "behind" you. Combine that with a lookahead to see if there's a letter ahead of you, and you get this:

(?<=\d{4})(?=[a-z])

Notice the dotted lines on the Debuggex Demo page. Those are the points you want to split on.

In PHP this would be:

var_dump(preg_split('/(?<=\d{4})(?=[a-z])/i', 'ABCDE1234ABCD1234ABCDEF1234'));

Use the principle of contrast:

\D+\d{4}
# requires at least one non digit
# followed by exactly four digits

See a demo on regex101.com.

---

In PHP this would be:

<?php
$string = 'ABCDE1234ABCD1234ABCDEF1234';
$regex = '~\D+\d{4}~';
preg_match_all($regex, $string, $matches);
?>

See a demo on ideone.com.

You don't want preg_split, you want preg_match_all:

$str = 'ABCDE1234ABCD1234ABCDEF1234';
preg_match_all('/[a-z]+[0-9]{4}/i', $str, $matches);
var_dump($matches);

Output:

array(1) {
  [0]=>
  array(3) {
    [0]=>
    string(9) "ABCDE1234"
    [1]=>
    string(8) "ABCD1234"
    [2]=>
    string(10) "ABCDEF1234"
  }
}