I've found an issue with over_voltage=4 (or higher) on Pi2.
Can you test this firmware
Unzip and replace start.elf/fixup.dat on boot partition with start_x.elf/fixup_x.dat from zip file.
Let me know if over_voltage=4 starts to work again.
Hi just tested,replaced firmware and can confirm I can now set over_voltage=4 or higher..but had to remove "_x" from firmware file names otherwise it didn't boot.Thanks.