I own 4k TV's so I get recording in 4k makes sense, however any reason not to on the iPhone @30fps? I don't care about file size, 128GB is plenty for my phone of quick videos of the kids and pets. Would i be better at 1080 or 4k? Thanks in advance!
For quick motion, stick with 1080p/60 for the faster framer ate. For everything else, use 4k. If you don't plan on panning much, 4k will also be okay for some slower motion subjects. Just do some testing and see what you like. Obviously 4k is going to look best on your 4k tv at native resolution.