4000 BC? I don't buy it: if you look at how little Greek changed over the last 3500 years and compared to how different it is to other languages this doesn't sit right
Because that difference in expansion, distance, and interaction with other cultures might explain the differences you're incredulous about.
Given Punjabi and Irish Gaelic are both descended from PIE, as is _Greek_ I'm going to say that 4K is plenty. The fact that Greek didn't change so much after Mycenean Greek was first attested in 2K BC doesn't say as much about PIE as it does about the Greeks deciding they liked where they were.
Also, _how_ different is Greek to other languages? Given it's a PIE language also.
Still there’s a lot more intelligibility than between English and Old English for example, which is a good example of how much can change in about 1000 years. Similar with French or Romanian and Latin.