> Those on 3g will have to wait ages for the video to buffer. Those on an old device won't be able to play it at all if you used new codecs.

This advice is pretty stale: 3G is rather old by now – 4G came out in 2009 – and unless you’re still supporting IE8 HTML5 makes it easy to let the browser select from multiple sources (subtitles have similarly had wide support for at least half a decade). Properly hinted video will start playing pretty quickly — certainly not massively slower than someone with 3G on an old device is going to experience everywhere else, especially since that old device isn’t going to pick the 4K version anyway.