It's in terms of video that the GH4 really stands out. It produces some of the best video we've yet seen - losing out only to the Sony a7S's moiré-free 1080 output. The ability to capture good quality 4K, whether for use at full resolution, downsampling to 1080 or cropping to 1080 adds real flexibility to the camera. Low light performance is solid if not exceptional - so you'll need to think about lighting and bright lenses for low light shooting - but in many situations the GH4 produces good footage with a little subject/background distinction.
The biggest single advantage of the $1600 Panasonic GH4 is how complete it is straight out of the box and how affordable it is relative to professional cinema cameras. The GH4's video capability is well specified even in comparison to professional cinema cameras upwards of $10k. Although there are many reasons why outright specs are not the whole story, the GH4 has found favour in the professional filmmaking community because it's small and simple to capture 4K with. Consumers stand to benefit from this, but adding all the features of a professional video acquisition workflow to the GH4 takes some building blocks. The appeal of cameras like the Canon C300 at $15k to pros is that the blocks come integrated and ready to use out of the box. XLR audio interfaces, ND filters and a top handle are examples of features the GH4 needs to be outfitted with whereas often more expensive pro video tools like the Canon C300 often don't (although often cinema cameras like the Epic and Alexa, conversely, do).
However despite it being a bit of an apples to oranges comparison with the Canon C300, the GH4 benefits from a smaller size, smaller lenses, lower weight and the 4K image, with efficient long-running internal 4K recording, a 10bit 4K HDMI output and even a slow-mo mode all for $1600 whereas the C300 has none of the aforementioned imaging capabilities even at $15,000.
Few cameras can go very high ISO and not suffer drastically. A7s is the exception to the rule, and sets new standards.. but C100 is ok, c300 is pretty good, 5d Mark II was alright, but personally wouldn't use 3200iso, 5d Mark III I'd push 6400 at the absolute most.
The gh4 in 4k can do 3200 pretty competently when downsized, 1600 is fine in 1080 too... The grain it does introduce is exactly that... grain... not macroblocked **** like the canon DSLRs do and as do most cameras.
