Yes, actually 4mm does seem low as I recall thinking it was approximately 0.25", so I believe I was told 6mm.
There is however a physical limitation, and that probably varies between people too. Also I'm quite sure that the law of diminishing returns applies here. Can you take more bone off past what is considered safe? I'm sure, but how will the soft tissue reattach? If you have soft tissue along your jaw, where is it going to go--it's going to hang down, it doesn't disappear. You can do a facelift but I suspect there will still be pockets of sagginess hanging around (and you can see this is some before and afters, even in younger patients)--this is why almost all surgeons are not super aggressive with jaw contouring, if it made sense to do 8mm+ reductions why wouldn't more surgeons do it? I assure you it's not because they are intentionally trying to be conservative.