I know for sure that Mac Pro late 2013 supports 4k screens flawlessly, but is HiDPI supported?
My understanding is that when Apple is saying that a high DPI display is officially supported, they are implying that HiDPI mode is supported. See, for example, that in the article officially stating support for certain 4K displays for your Mac, they show a screenshot and state that you should be able to adjust scaling:
E.g., the Dell monitor cited in the article is quite a high pixel density since it shows 4K in a 24 inch diagonal. Running it without HiDPI would be quite challenging for most users.