but a "400 mile" battery is very likely not 400 highway miles. Figure a 500-600 mile battery to get your 400 highway miles. At the current rate of capacity increase those cars are only 5-8 years away.
Batteries are not rated like fuel tanks. A "400 mile" battery (per mfr spec) is probably good for 300 miles after a couple of years. At freeway speeds, that's probably 200 miles. If your freeway limit is 200 miles, you need a way to recharge it after 150 miles or so.
In other words, you really would like an 900 mile battery (per mfr spec).
Might be 10 years away.