about 1/24 of the 25k-mile circumference is the Earth, so 4 time zones should cover about 4,100-4,200 miles. But the continental US is only about 2,800 miles wide, from East to West. IOW, the continental US should probably only have 3 time zones, rather than 4.
So if you want to have 30-minute time zones, rather than 1-hour time zones, we should therefore probably have 6 of those.