As far as I understood you’ve sliced them 64x64. Now you’ve scaled them x4. So now they should be 256x256.
Press “Slice” button on top-left corner of your Sprite Editor. Now you have 3 options, either automatic + sett your settings and press slice OR yourself (grid) then set pixel size to 256x256 + sett other settings and press slice OR if you’re not happy with that you can manually size all of them by selecting each one and resizing and positioning properly, though I can see no reason why would you do that.