The amount of gallons of water that are in 4 inches of rain depends on how large of an area the 4 inches of rain falls on.
But for example for every 1 inch of rain that falls on 1 acre of land there are about 27,154 gallons of water in that 1 inch of rainfall.
So if you have 4 inches of rain that fall over 1 acre of land you would have 108,616 gallons of water in 4 inches of rainfall that falls over a 1 acre land.
If you were to capture all that water you would need some large tanks.
But of course if the rainfall falls over a less of an area such as from your roof you would get around 2,000 to 3,000 gallons of water in the 4 inches of rain depending on your roof size.
And of course if the rain falls over a larger area than just 4 acres you'd have over a million gallons of water from the 4 inches of rainfall.