From north Sweden
I'd get the Ursa Mini 4,6 if I were you!
I've used the GH4 a lot and even though you are doing night shots I guess you will be lighting your scenes too? Then it should be no problem.
The 1080/60p All-I is pretty nice in the GH4 and with a Speedbooster you can really push the iris.
A7s would probably be a good choice for low light but I would use something less wide than 15mm on a full frame.
I have a 15" MBP Retina which works fine with 4K Prores when you're on the fly and not doing too much heavy stuff. For bigger more demanding projects I'd recommend using a desktop.