Think and answer

Out of the 7 cows taking 7 days to eat 7 grass fields, let's number the cows and the grass fields that they eat. Cows 1 through 4 will eat grass fields 1 through 4 in the same 7 days.

If 7 cows take 7 days to eat 7 grass fields, then:

1 cow takes 7 days to eat 1 grass field (1 cow eats a seventh part = 1grass field during the same period of time) , and therefore

4 cows take 7 days to eat 4 grass fields.