Hello Freeman,
With all leading Zeroes your actual MATNR is 000000000123456789. Now you want to extract 4567 means 4 characters after 12th character. Crete an Additional field say ZMATNR of type char4. Now give the code below to this ZMATNR
CLEAR ZMATNR.
ZMATNR = MARA-MATNR+12(4).
This will store 4567 into ZMATNR. You need to do this exercise in Infoset (SQ02) Extras > Extras. You may refer to part-1 of this document on how to create and give a code to an Additional field in Infoset.
Good luck
KJogeswaraRao