Two comments on this: 1) The way I read your post was "How can the attacker block step 3?" That's not what the linked webpage is suggesting: "... the acknowledgement to message #3 can be blocked ..."

So really this is blocking step #4 from occurring, pedantic perhaps, but it's sort of important.

2) In response to "How can someone block step #4?" Since we're discussing Wifi then signal jamming seems apt. From the actual paper discussing krack: "Inspired by this observation, an adversary could also selectively jam message 4 ..." There's much literature here (google). The reference the KRACK paper provides is: "Mathy Vanhoef and Frank Piessens. 2014. Advanced Wi-Fi attacks using commodity hardware. In ACSAC" link here: [https://people.cs.kuleuven.be/~mathy.vanhoef/papers/acsac201...]