From: United Kingdom
Looks like the Rewrite speeds are 4, 10, 12, 16, 24
4x can be the top speed of 1-4x media, or the lowest speed of 4-10x media, and the calibrations are VERY different.
Where a drive does not support 2x for CD-RW, it generally does not support old-style 1-4x media either, even at 4x.
Later HS-RW (10x/12x) drives tend not to like 1-4x media, even if the do manage to write it.
If you have acquired a large stock, I'd suggest ebaying it in suitably sized bundles, for people with older drives that can't take HS media.