In terms of whats happening with the teams at Tesla, it certainly looks like a lot of churn, but it's hard to know the real severity of it because of the over reactions by the press.
4 years at a company is a good amount of time. I'd be worried if the Head of Battery at Tesla was a super star engineer who joined with a lot of fanfare and then left the company in less than a year or two (e.g. Chris Lattner joining Tesla and leaving after a very short stint).