Sorry, how are you getting four attacks there?
I think the four attacks in that build are coming from:
1. Two attacks from extra attack at Barbarian level 5.
2. And two bonus attacks which are coming from having two bonus actions as a Thief level 3.
Correct, and all with Advantage.
1. Reckless Attack
2. 2nd Attack
3. Bonus Action Attack Frenzied Strike/melee or Ranged
4. Bonus Action Attack Frenzied Strike/melee or Ranged
5. Helmet of Grit Bonus Action attack Frenzied Strike melee or Ranged
Also with +10 to damage with GWM and with a Great/two handed weapon and 21+ strength.
The damage numbers will be insane.
