Regarding x4 extender slippage, the brief answer is Bulldog clamps.
Before I was able to get on here, I only had access to the x4 message boards and scanned them thoroughly, having sometimes to go back several years to get the answer I needed. I had all the typical extender problems, one of the key being the comfort band (HA HA HA what a misnomer) slipping. In the forums, and you will probably have to search to find it, somebody mentioned bulldog clamps,which you can get from any office supply store or probably walmart or target. They are the clips that you use to secure large piles of paper together.
The smallest ones around 3/4 inch wide will work adequately. Once you tighten the strap down, you simply attach the clamp on the back side,(1 on each end of the strap) preventing most of the slippage.
The comfort straps do have a limited lifespan however, and after 4 or 5 months, should be replaced as even with the bulldog clamps, they still can slip. Hope this helps.