You will be teleported back to the upper floor of the helmet shop

In his fourth form he’ll start attacking extremely rapidly. However, most of the damage that he does is not very critical. You won’t have Airrivals Spi to kill him for the fourth time. Just make an attempt to kill him.

If you climb back up the ladder during the battle then your character will have to forfeit. Talk to Thorvald again for a rematch.

Place your Garlic on the pipe next to Sinister Stranger. He’ll then move to another location to fish. Fish at the location where the vampire was at (the northern most fishing spot) and wait until you catch 3 carps. Bonzo will announce you as the winner and hand you the trophy.

Once he kills you (or you kill him), you’ll be teleported back to the upper floor of the helmet shop. As long as you’ve managed to kill Kochei in his third form and not retreat from the battle then you’ll get Thorvald’s approval.

Also, if you’ve managed to beat Kochei all 4 times then Thorvald will give you a Fremennik sword as a reward.

Now, head back to the building where Brundt is in and talk to Manni the Reveller. He’ll approve of you joining the tribe, but only if you can beat him in a drinking contest.