4 cups can equal 1 gallon if the size of the cups are 32 ounces each.
However if you have an 8 ounce cup then it would take 16 of those 8 ounce cups to equal 1 gallon.
There are always 128 ounces in 1 gallon.
So to find out how many cups it would take to make 1 gallon you need to know the size of cup you're using to equal a gallon.
Some cups are 8 ounces, some cups may be 16 ounces and if you do have a large enough cup that is 32 ounces then it would only take 4 of those 32 ounce cups to equal 1 gallon.
But for an average size 8 ounce cup it takes 16 of those 8 ounce cups to equal 1 gallon.
Just remember that 1 gallon contains 128 ounces.
So take the size of the cup in ounces and multiply that number by the amount it takes to make 128.