Yes, that is normal for the 4163B. It reads single layer pressed DVD movie discs at max 10x speed and pressed DL movie discs at max 8x because of riplock. DVD+R/-R discs are read at max 10x as well. The only discs is can read at 16x are single layer pressed DVD discs (e.g. a game which comes as a DVD disc).
Oddly enough, the drive can burn at 16x. So you have a drive that actually burns faster than it reads.
Usually the read speed at the beginning of a pressed SL DVD movie disc or DVD+R/-R disc is always about 4x and it speeds up to 10x at the end. So, yes what you are seeing is normal. I don't think there are any drives which actually start reading at the start of the disc at 16x and stay the same speed all the way through.