The way I understand it is the Pioneer ones have a table in the firmware that determines what the max speed is for each media. For the Pioneers before A05/105, there are hacked firmwares to let you write to disk at high speed. But Pioneer claims that some drives have been/will be damaged. That is why some DVD-Rs can be written at 2x/4x on Pioneer burners, but not on the A05/105s.
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The ONLY drive Pioneer makes that will burn at 4X is the A05/105! The A04,with hacked firmware could burn 1X media at 2X never at 4X.There are some reports you can fool the A05 into burning on some 2x media by letting it see a 4x certified disc,and then switch it.Also be aware not ALL 2x media will burn at even 2x on the Pioneer.Unless Pioneer has aproved it,and entered it into their database,it probably will only work at 1x.The part about damaging the earlier drives with 4x media,is partly true!They claim using 4x media(not at 4x) in the A04/A03 can damage the drives.Pioneer has released a firmware upgrade to fix this.They also state it may damage some home players using backups burned with 4X media.
I want to add,I also have the Sony 500a.I have never had to burn a 2x rated disc(with F firmware) at 1x! Just the opposite,I can burn lots of 1x media at 2x! This includes Accu and Lead Data.I could buy 150 Lead Data for $102.99 shipped! Out of 600 burns with my Sony,close to 500 were with these 2 medias.I ca say I have had some coasters though.I had 3 discs I clicked record before I was ready!