To everyone who has had trouble with the K4 lumbar pad slipping. I had a small breakthrough on my K4. I have been switching back and forth between a K4 and a Kifaru tactical doing sandbag training. I am trying to see which I prefer. I had been having some difficulty with the K4 lumbar pad riding down and causing pain on my butt. After watching the last video by K4, regarding the 3 mistakes (thank you Exo for the great instructions!) (I watched all the fit videos previously, some more than once), I realized my problem with the lumbar pad was actually caused by my load lifters. When I was re-fitting the K4, trying to dial it in, I realized if you over tighten the load lifters, the way the frame sits (at least on my back), it would actually pull the frame at the top towards my shoulders and would act as a fulcrum causing the opposite effect on the lumbar pad. It essentially attempts to pull the lumbar pad off my back as the top of the frame moves closer, the bottom moves away. It isn't dramatic meaning the lumbar pad is not touching my back. I still have lumbar pad contact, but it is enough to cause the pad to slide down. It was in the fit video, and I missed it the first time, but only tightening the load lifters to tension is key. I was over tightening. The effect is more pronounced as you load the bag up. It's not noticeable empty, but under my sandbag load of 60lbs I could feel tension trying to pull the lumbar pad off the back. I think this is due to the way the carbon fiber stays are curved. I don't have a super curved back, so maybe this effect is more exaggerated for some body styles.
Anyway, after changing my load lifter tension and really only tightening to remove slack, maybe a bit more, I had a much better experience with the lumbar pad sitting nice. I didn't feel slippage. My neighborhood training run was the best so far. I'm still tweaking things hear and there, but it's much better.
Perhaps those who struggled with slippage might evaluate the load lifter tension. I don't think this is a silver bullet for all the slippage issues that each person might have, but it could be a partial cause. FWIW I prefer a larger and firmer lumbar pad as well. Before recognizing my mistake I was looking to make the K4 lumbar pad more firm and larger, which I now think was trying to compensate for the pull away from my back.