4 a.m . in the morning San Fran time at the time you depart Boston at 8: am. If for example, this is a 4 hour flight, you will arrive at San Fran at 8:00 a.m. .local time – same time you took off in Boston.
Check a time zone map if you are not sure how to do this. Boston is ahead of San Fran by 4 hours because of the way the Earth rotates – counterclockwise when viewed from the North Pole. This means that when, say is just coming up in Boston, its still dark in San Fran.