1) Yes, the first 4 characters will always be letters.
2) It may have different letters in the future.
3) No its not hex, this is just data that is stored in the database. Its just text.
4) The last 8 characters is always intended to be part of the string.
The field in the database is VARCHAR(12) so it wont ever be longer that 12 characters.
Given those conditions, substr is indeed the appropriate function.
$letters=substr($lastADID,0,4);
$numbers=substr($lastADID,4);
To recombine later, simply put a '.' between them. Implode will work, but it is overkill for combining two pieces.
$recombined=$letters.$numbers;