As 2009 mac pro (4,1) can flash to be 2010-2012 (5,1). And it hasn't different to the other two. Now, apple state that, macOS Sierra has support only 2010 Mac Pro and later. So now there are two option for macOS installer to handle 2009 Mac Pro which flash to 5,1. 1 Define it as 5,1 mac pro and let the installer act like 5,1 mac pro 2 Define it as early 2009 mac pro and block the installer. Do you guy think what is the way apple choose?