I started doing WT4 at 65 and at 67 can join world bosses and they go down pretty fast. I was actually a bit disappointed how fast a legion event boss died and we were all 70ish at the time. Was hoping open world stuff would stay hard since 4 is the last tier we got. Thankfully nightmare dungeons have the modifiers though, that crap gets really hard past tier 21.
btw if you want some really good stuff, just go to wt4 during a helltide and follow some people around grabbing cinders. then hit a mysterious cache for massive gear increase.