I'm hesitant to recommend something. I don't know your specific packing system or what might be best for your body. But some general thoughts you might follow up on, here or at Rokslide, etc.:
1. I haven't tried the K4, but it has a lot of flex towards the hips and Exo has a reputation for a softer feel.
2. Honestly, for me, the Seek Outside Revolution frame doesn't need a lumbar pad at all. The frame design is sort of reverse of how others do it, being wider at the base. For me at about 38" waist, it's a great fit.
Seek's integrated packs basically use the same frame design. Search up "Seek Outside Integrated Vs. Breakaway Backpacks (Main Differences)" on Youtube and they explain it all. You might hate their bags though. They got away from cordura and, yes, custom builds a few years ago and moved to waterproof Ultra PE and Spectra fabrics (Now they use Ultra-grid vice Spectra due to supply issues). This makes their bags awesomely waterproof, but noisier than cordura and not camo. (I haven't seen Ultragrid but its sail fabric so, likely very similar to Spectra.) Seek's external Revolution frame is pretty easy to bag to...I've seen where folks are mating Kuiu bags to the Revolution frame with very little work. It opens up options.
Seek stuff wins in ultra light.