While Full Frame DSLR cameras have the advantage on the wide shot, these mirrorless cameras have advantages for distant shots due to it's crop factor. As an added bonus, the Ex Tele Conversion mode in these Panasonic cameras switch to using pixels in the center of the sensor creating a further cropped view (I believe an additional 2.6x crop). Since it's still using a 1:1 pixel readout, there is no loss in resolution for full 1080p video.
This is extremely handy as you can easily more than double the focal length of any lens. With this mode, my Lumix 12-35mm F/2.8 (24-70mm equivalent) quickly turns into something like a 31mm-90mm F/2.8 [60-180mm 35mm equivalent], and all without losing a stop.
Having so many options in a single camera is new to me, so i'm still learning the best ways to use the GH4. Knowing that I can crop in on a 4K video file is a great option, but is it better than using the EX Tele Mode at 1080p? I shot this very short clip to test this out, let me know what you think.
Certainly cropping from 4K still retains plenty of details. To the average viewer, they may not pick up on such subtle differences, but judging from my eyes I think the Ext Tele Mode in 1080p is better. If i'm considering my final output to be 1080p, I can use this mode with the 35-100mm F/2.8 (70-200mm equivalent) essentially giving me something around a 90-260mm F/2.8. Remember that if you compare this to 35mm equiv it's like shooting with a 180-520mm F/2.8 on a Canon 5D (but for a lot less money).
Keeping in mind that you double the focal distance on an M43 lens to get that 35mm equivalent, I decided to grab some samples from my new Lumix 100-300mm OIS which is 35mm equivalent to a 200-600mm. With Ex Tele Mode this lens should be around 260mm-780mm which should be like working with a view similar to a 520mm-780mm - again without reducing the amount of light like a teleconverter extension would on a Canon 5D.
Here's another short clip testing out cropping from 4K as opposed to using EX Tele Mode in 1080p with my Lumix 100-300mm OIS lens.
Lumix 100-300mm F/4.0-5.6 OIS Lens
Here's a few other GH4 clips from my trip to the Academy of Science in San Francisco yesterday. Nothing special, just me learning.