It's not 4 challenges.
It's 3 challenges and 1 extra challenge at the start of a tie break, the same as used at the Grand Slams. The start of the article is misleading.
Is it just me or did the umpire make a mistake with Karlovic's challenges number in the 3rd set. Not that it mattered because he didn't use it but after Ivo's second incorrect challenge the umpire said Mr Karlovic has one challenge remaining. Could have been quite important. 4 challenges is quite a lot though. I think we'll see them used more if they get that many.
Surely if he had 2 incorrect challenges then he would have had 1 left......