The BenQ does not support burning speeds below 2.4x
If all the options you get are 4x then that is all the burning speeds the firmware supports for that media. There is no way to get this any slower without swapping codes around and this then gets messy.
Burning slow does not give you a good burn with newer drives and media. If I burn my MCC004 media (16x +r) slower than 8x I end up putting them all in the bin.
Why do you need to burn so slowly?