Aes Dana - Memory Shell - 2004
Anja Schneider & GummiHz - Back To Back (Remixes Part 2) - 2009

2004 - Aes Dana - Memory Shell
2009 - Anja Schneider & GummiHz - Back To Back (Remixes Part 2)

rename -n 's/(.*) - ([0-9]{4})//$2 - $1/' */

$ tree
.
├── Aes Dana - Memory Shell - 2004
├── Anja Schneider & GummiHz - Back To Back (Remixes Part 2) - 2009
└── rename_dirs.py

2 directories, 1 file
$ ./rename_dirs.py */
$ tree
.
├── 2004 - Aes Dana  -  Memory Shell
├── 2009 - Anja Schneider & GummiHz  -  Back To Back (Remixes Part 2)
└── rename_dirs.py