The 4-digit code is always derived from the 7-digit code internally. Therefore, if you have software that uses the 4-digit code, it's preferable because it can be used consistently. Furthermore, the 4-digit code is always transmitted via the cable.
By the way, the EEPROM of EDC15 and ME7 systems can also be completely read out via diagnostics.
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http://www.heise.de/newsticker/meldung/58874
I can easily imagine that such links might cause problems later on, especially if people have completely forgotten about the original post.