As already said, flash drive and HDD are not advised, they can break too easy, so I won't count that much on them. A piece of laminated paper is already better. Steel would be even better.
But from what you wrote here I have the feeling that all "four separate locations" are in the same house. Maybe I'm wrong, still, I would like to emphasize that for life changing funds you should use geographically different locations. I mean that if your house/area/city is ever devastated by fire, or hurricane, or whatever, you may need a proper backup somewhere else.