yes you can CT4100 fw to dr hd d15 but the OSD Menü Background is not same wirh org CT ! 
Load the Hex Editor and open the CT fw on this and edit on the 2. Line 0x000010> 34A301 <<< see pic http://i43.tinypic.com/xbbjw3.jpg
save this on the pc and send it with ctchup to the DR HD D15!
but ist work only on PC with RS232 Cable or RS232 to USB Cable!
Load the Hex Editor and open the CT fw on this and edit on the 2. Line 0x000010> 34A301 <<< see pic http://i43.tinypic.com/xbbjw3.jpg
save this on the pc and send it with ctchup to the DR HD D15!
but ist work only on PC with RS232 Cable or RS232 to USB Cable!