"Reading devices only reading slowly record media are a myth"
This is probably very close to the truth.
At the time when newer "fast" DVD-Rs appeared, most of them gave questionable quality burns on the existing burners when higher speeds were used. Thus the word was spreading that lower speed burns gave better results. Flawed logic. The real issue was not the speed but the media.
Nowadays, this is mostly not true anymore (except for crappy media of course), but and I think the best choice for console owners is to burn 4x specified medias, that consoles like, instead of performing @4x burns with 8x or 16x media that consoles may like or not...
So ruzinho, you probably choosed the right media for your needs
Many high-speed media don't like to be burnt at low speed and this can actually worsen the console issue.
The only high-speed, quality media that I know of which actually gives perfect results @4x is MCC03RG20. This discs are simply amazing, they seem to be the most versatile discs on the market today. I LOVE them.