Really the easiest thing to do is just switch to T4 and try it. It’ll show you where your weaknesses are.
When I first got there in my barb I did a few nightmare dungeons. I got it done but it was slow so I switched back to T3 leveled my glyphs more and did more masterworking. Switched back again and gave it another shot. It was faster this time so I tried hordes. Got like 200 aether so again I switched back to T3 did some more work.
Now I am able to stay in T4 and do all content pretty effectiently. The point here is You don’t have to wait for some defining moment you can switch back and forth as need.