let 'x' represent my age, and 'y' represent the number of years ago that you were my age

you were my age 'y' years ago:

40-y = x

'y years ago' I was 'x-y' years old and you are now 4 times that age:

40 = 4(x-y) = 4x-4y

we substitute 'x' with '40-y':

40 = 4(40-y) - 4y

solving for 'y' we have y=15

knowing 'y' we can calculate my age:

40-15 = 25

I am 25 years old