Ao4
 

Is it possible to mod the Pioneer Ao4 by means of a firmware upgrade to burn at 4x

NO!!:)

2x some media yes with un-official v1.31
4x no

2x all all media is the best u can get

problem being some media plays badly as it cant handle 2x

firmware 1.31 is best and stick to riteks at 2x :)

http://firmwares.co.uk/

allows u to downgrade your firmare
has 2x write all patch,firware1.31 official but never released(2x on ritek)

multi region drive as well is available