I must disagree with what 90% of tech forums say on this issue. They don't understand what 4K or HD video even means.
I would recommend shooting in 4K unless you plan on displaying it directly on a 4K high quality TV.
FaceBook, YouTube and similar websites still can't handle AVCHD yet out of left field they claim they can handle 4K which is three levels higher? People should be smart enough to know this is an outright lie. These websites will down convert AVCHD and any higher format to a lower version of 1080.
Even compressed won't it still come out in higher quality? Maybe not. Focusing is important and if your 4K video is out of focus it won't look as good as your 1080 video which is in focus.
AVCHD is already super high quality. So if it's a once in a lifetime video it will still be obscenely high quality. This is of such high quality it can only be properly displayed on a 4K TV.
AVCHD is a lot more user friendly as it can be played on any TV or PC. (the rule still holds that FaceBook will down convert this quality)
Still want high quality? The A6300 also does the next level up, XAVCS! (the rule still holds that FaceBook will down convert this quality)
In my opinion many people are going out of their way to shoot in 4K only to have it down converted by THREE LEVELS to upload to FaceBook. It's like shooting with color film only to print it out in black and white.