Reducing DVD Write speed with Nero 6 on Sony DVD writer

I have Nero 6 and a Sony DW-D22A DVD writer. Writing on 1x or 2X blank
DVD's produces perfect results, but when I use Fuji 4x DVD-s I get
write errors. I'm trying to reduce the writing speed but Nero does not
let me do that, it recognizes the 4x DVD and the speed selector drop
down is grayed out with a fixed 4x speed.
I've seen similar posts with this question but no answers. Does
anybody know how to reduce the DVD write speed in Nero?

Thanks,
Guido

Guido