Welcome to the Forum.
Exactly why do you need to specifically burn your Sega Dreamcast game disc at 4x/8x speed?
With a modern DVD Burner and with modern quality error free Media you should be able Burn modern quality error free Media at or slightly below the Media rated speed.
With a modern DVD Burner and with modern quality error free Media there is no advantage in Burning Media at excessively slow speeds. In fact better quality Burning results are obtained Burning modern quality error free Media at or slightly below the Media rated speed.
Unfortunately you failed to state exactly what software Burning program you are attempting to use but suggest using the ImgBurn (http://www.imgburn.com) software Burning program to Burn your Sega Dreamcast game disc.
Also ensure that you are only using known proven quality error free Media such as Taiyo Yuden or Verbatim Media which has an excellent proven track history of consistently producing quality error free results.