I haven't worked at Google, but I have interviewed with and reviewed feedback from Xooglers, and 4's are not impossible -- but they are rare and you have to essentially be smarter than your interviewer about the particular problem to get one. This can be pretty hard, since many interviewers pick problems that they are extremely familiar with.
On my Google onsite (in Zürich as well), one of the interviewers told me that if he gave me a 4.0, he'd have to argue on my behalf if the hiring committee wasn't sure of hiring me.
Arguing to HC on behalf of someone I gave 4.0 (which is rare, and which I did give a few times) is something I would do gladly (and I expect most others will do as well).
It's not like going in front of HC is an inquisition or anything -- they are just your peers.