I've had mine a couple of days now, and have about blinded myself sitting about three feet from the 55" UHD screen showing clip after clip through the camera's HDMI out. My wife thinks her mate of 52 years, sitting there spellbound in his gallery glasses, has totally lost it! OK, over that now, and on to a couple of observations:
First, 2X dig. zoom in UHD mode is identical to 1080's ETC mode in mag., but enhanced by being on-the-fly uprezzed to full UHD frame dimensions as shown by frame captures compared to FHD ETC mode captures. The former are a bit sharper, smoother, with less artifact than the latter at the same 100mbps bit rate. A nice straight OOC enhancement, with a hardly noticeable drop in IQ at 6-8 feet viewing.
When watching 4K through HDMI out in the TV's 1080p mode, the GH4 auto down-converts the video with results similar to what we have seen with PP conversion on the web - quite a difference in resolution and gradation compared to native 1080p clips shot at the same bitrate. This supports Adam Wilts' recent findings that native FHD from DSLRs doesn't come close to 1080 Lines/ph in resolution - only about 700 at best, or 800 for the GH3/ETC 1:1 mode. But that down-converted 4K hits the full 1080 before aliasing sets in.
Last,I highly recommend Andrew Reid's GH4 4K video Shooter's Guide ($20) with his personal take on initial settings, a "special sauce" version of CinelikeV settings for very nice OOC results indoor and out, plus his versions for grading and ultimate flatness. A very comprehensive discussion of speed-boosters, crop factors, legacy lenses of all sorts, and an extensive reference section.
http://www.eoshd.com/eoshd-panasonic-gh4-shooters-guide