For JPG, you can definitely get good quality results with Save for Web at around as far down as about 55%. You just have to keep an eye on the details you want to preserve and adjust accordingly. 

As far as PNG's go, with or without transparency, it makes little difference. Since there is no quality slider for PNG in Save for Web you can use a third party PNG option like SuperPNG. While it will probably have little effect at 4k with lots of color data [again, the trans is irrelevant] it can shave off a few megs as compared to a direct [default] PNG save or Save for Web PNG save.  

SuperPNG really only shines when you want to use a lot of PNG's and they are small - it's the cumulative effect - at least that's been my experience.