4x was the goal or standard for simple cyanine-based media until 2001 when Imation led the industry to adopt azo-cyanine which is presently optimized for 12x. So-called archival discs may use phthalocyanine, which they will tell you not to burn at less than 8x. Verbatim pushed super azo in 2002, which is optimized for no less than 24x and now the Japanese plants that make most of the discs do runs with different dyes for different manufacturers/tolerances, but to my knowledge, nobody makes a simple cyanine-dye CD-R anymore so burning at 4x is ill-advised.

Of course, your drive may be old and the laser quite weak, in which case you won't be able to burn anything but coasters at high speed. That's not an issue with the media.

You may still find some sites refer to 4x as an optimum burn speed, but they are ancient.