This release is for calibre 5.x. It includes the DeDRM and Obok plugins.
For calibre 4.23, you must use the 6.8.1 DeDRM plugins but the included Book plugin should work.
There may well still be problems. If you find a book that can be decrypted with calibre 4.23 and DeDRM 6.8.1, but that does not decrypt with DeDRM 7.1.0, please send me to original encrypted book, and your key exported from the DeDRM 6.8.1 preferences. I will try to reproduce the problem and fix the bug. Please send books and keys to firstname.lastname@example.org
Please DO NOT send me anything unless you can get it to decrypt successfully with calibre 4.23 and DeDRM 6.8.1