The 1070 is faster and TBH its not even good enough for solid 60 fps 4k gaming without the details turned down a bit. For 4k you need the fastest you can get so as I said of those 2, 1070.
get the 1070, it has more VRAM which is useful for those 4k textures.
you will definitely have to turn down some settings but at 4K you they will be less noticeable than 1080p. the resolution jump will already look nice enough for you.