As usual, it depends.
And there are several categories of media & write...
16x @ 16x - supported 16x media, some drives are so well tuned to one media brand/ID that the result is beyond reproach - others - UGH!
16x @ 4x - If you get that, the drive is defaulting to 4x, as it doesn't know the 16x media.
8x @ 12x - Some drives will overspeed quality or well tuned 8x media to 12x, and make a decent job of it... others try...
8x @ 8x - with good media on a good drive, no reason for this to be any worse than 4x
Of course, if your drive overspeeds an 8x media to 12x, you can always hold it to 8x if you prefer.
Inevitably, 4x media production will be phased out... since 8x media normally has to start at 4x in an 8x drive (Zone-CLV mode), 8x media should work in a 4x drive if the firmware supports the code. If it falls back to default, the default on a 4x drive is likely to be 2x for -R and 2.4x for +R