External monitor + power at once. I highly doubt that battery's going to drive a 4k for 8+ hours. Actually, I doubt it can drive a 4k @ 60hz at all but mostly because Apple has not released the full specs even though it's been over a year since the Macbook has been released. Even @ 30hz though or 1440p @ 60hz, this can often be taxing on an MBPro let alone the regular Macbook. There might be an adapter that provides both power and Displayport 1.2 or greater, but after months of research I stopped looking. I can barely find regular USB C cables and accessories that people have had good luck with as it is.