You’ll need to locate your spreader’s owners manual to find the setting that applies 4 lbs. per 1000 sq. ft.  No watering is necessary.  That said, I’d not feed the lawn during this period of high heat.  Water only until temps. drop back into the 70°s.   Feeding during the heat of summer doesn’t help a lawn that is already under stress.  Water heavily applying an inch of water all at one time once a week for deep watering…a light spray just evaporates before reaching the roots.