First up, don't post about the same problem in different threads. It confuses people who are trying to help you with your problem because they don't know which one to follow. It is also against forum rules.
You only listed the MID for the 8x stuff so I can mostly only comment on that.
I suspect the companies behind those discs probably does not make its own media but buys its media from various media manufacturers and then just prints its brand on it. It also switches manufacturers around from time to time. So the discs may look exactly the same but the media codes from one batch to another may be entirely different. If this is what happened, then it is possible that one batch of media was actually 8x stuff and the next batch 4x stuff.
Based on the MID you posted above, SKC Co.,Ltd. appears to be really only 4x rated media. I checked this with the firmware support of 4 older writers just to make sure. So it is nowonder that it only burns at 4x! Can you check the media codes of the previous discs of the same brand that really managed to burn at 8x?
Also, please post the MID of those WRITEX 16x discs. It's not possible to know what is going on without that bit of information.