When do you think will there be a price drop again for HDD?

I wanted to change sooner or later all my HDDs in my NAS (6x2TB), and I wanted to put 6x4TB. But at the moment the price for a 4TB HDD is still around 200€.

Annoying thing is also that after the swap, you will end up with 6 spare 2TB HDDs, which you cannot really use anymore.