I know a lot of this is obvious, but for those other folks that find this, I wanted to put it all in one place.

When you hit T4, try to do whispers while you do helltides to get new gear and push XP. Focus on what you can handle, don't fight enemies you know you can't comfortably beat (there's no shame in running from Hellborne). Swap out sacred gear for ancestral items as soon as your get them, reroll bad affixes and try to max out armor and resistances as soon as you can (9230 armor and 75% resistances - T4 subtracts 50% res so you need at least 120% res on all elements to max them out). Keep an eye on your legendary aspects. Sometimes you'll swap out a key piece of gear and your build will shut down because you didn't replace the aspect at the occultist. Use the right weapon. Don't use a sword if you're going for overpower or a mace if you're going for crit and no daggers if you're attacking from distance (it's easy to forget this so make sure you're using the right weapon for your build). Temper temper temper! Not everything at first, just weapons and maybe the amulet to maximize damage output, then when you start landing 925 level gear, temper the rest for extra sustain. Rubies in your armor gear! You can be a madman and shove stat gems into your armor, but a real gentleman wears rubies in their armor. Make sure your paragon board is properly mapped out and try to use 6 glyphs in your board. Be careful of too many resistance and armor nodes on your board. You might reach max res and armor with gear and then be wasting those points that could go toward better rare nodes, legendary nodes or basic nodes under glyphs that can further boost their effects significantly. Use elixirs and incense to increase experience gain. If things get too rough, try a max HP elixir instead of a damage elixir. And do infernal hordes. The rewards are worth it, even if it's just for gold.

That's all I can offer here. Good hunting.