Its really simple, you import the clips into the timeline. If working in 4K make sure you are working on a 4K timeline, if 1080 then scale the image down and go about your edit from there. There is not much to editing it.
depends on your output. I mean if your going to do a 4K output then I just down sample to make it easyer to work in, but if you edit in 4K you can output HD and 4K.
Ok so I guess 4K is a bear to work with. Have you tried shooting DCI 4K (which is letterboxed?). I read it looked a little sharper which seems far fetched. UHD might scale better on consumer TV's being double the native resolution. First thing is to buy a new SSD though, thanks.
UHD and DCI 4K have the same level of Sharpness if thats what you call it. You cant get any more detail then those mode for the simple fact they are taken the image pixel for pixel off the sensor. Much like RED does, thats why the epic shoots 5K so you down sample in post for a cleaner 4K image. Not saying the GH4 isnt clean. I would put it up there along side the RED. Of course its only 8 bit for now. The 10 bit should open up the camera a bit. If it can be hack for Raw then that will help.