16 gigs is probably low for 4K... Have you tried to switch to software rendering in your project settings. And 4K is not enough info. What is the codec and frame rate. by any chance are any of your sources from a smart phone or screen recording?
well to be more accurate, it's 23.976 and it's known as 24p. I know I sound like a jerk, but sometimes these distinctions are important. You do occasionally see material that's actually 24fps and it ain't the same. You sound like you know what you're doing but if I don't ask, I may regret it. Is there plenty of space on all your drives? a minimum of 10% on all drives and 20% on your startup drive.... and they're formatted properly? What is your preview format in your sequence settings? You might try smart rendering: change your preview format to an high quality all Iframe format like prores 422hq. And render in sections from in to out. If the whole sequence renders, export with the "use previews" option checked. Please try these things in a systematic fashion and post back and we can figure out how to proceed.