very surprisingly they waited (as officially told) 4 days before reading them
What's so surprising. This happened late on a Thursday, no? So Friday to recover the bits and pieces and check out the externals of the boxes once they get to the lab.
Sat & Sun - weekend. It's an incident during a ground test, not an inflight incident with immediate implications for inservice flight safety.
Monday afternoon, they read the data.
Like I say, what's the rush? If they were to say "oh, we rushed to read the tapes ASAP and accidentally erased some data - which COULD happen - the finger pointing/conspiracy-theorists would have a field day. Rule #1 in accident investigation - take your time, don't destroy evidence.