The drive's firmware decides which writing speeds are possible. When a disc is in the drive Nero will let you choose between the supported writing speeds of that media in that drive - it won't let you choose any other speed.
If Nero doesn't let you choose anything lower than 8x, then the drive/firmware doesn't allow it.
Is there any special reason you want to burn at 4x? If it is because you have read somewhere that audio shouldn't be burned faster than 4x, then that is no longer true with modern drives and media!
I suggest you try burning at 8x and see (hear?) how it goes.