I'd put it 5, but I think it is a serious accident. TMI-2 is apparently rated 5, but I'm not sure if any deaths were attributed to radiation.
There is a significant release of activity - outside of containment.
Can nuclear power systems be designed to be safer? Yes, of course.
First - don't put EDGs in a location where they can be destroyed. If the EDGs had worked, we wouldn't have this event.
Second - realize that the Fukushima containment is Mk I, which was superceded by Mk II, then Mk III, and now modern containment systems. In the more modern plants, e.g., Mk III, the spent fuel pools have been removed from the top of the reactor building into a separate area. There were numerous improvements.
I would expect regulators to consider some retrofit of Mk I containments.