Hi. Your reasoning is very good and your answer is correct, but your work shows that you subtracted four from each side, instead of dividing.
If your brother knows his multiplication table, another way to convince him (with such a simple equation) is to say that 4m is a product. It's the product we get when we multiply 4 by some number. If we want the product to be 12, then we ask ourselves, "What number do we multiply by 4 in order to get 12?"
In other words, 4 times what equals 12?
Cheers :cool: