>>>Your formula looks right to me, but you are right it seems a little too complex for a fourth grader.
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>>They aren't supposed to invent algebra. Just to play out with numbers, try a few and see what they get, using common factors etc. Yes, it's a quadratic equation, but actually a very simple one which can be solved by a little thinking, trial and error, without ever mentioning algebra.
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>I see. My school days are a blur, and for the little I remember, I even have difficulties explaining my daughter basic stuff, as it is different as what I've learnt, for example some years ago she was learning division and I was trying to explain her that what she was doing was wrong, she had the numerator and the denominator in the wrong place, then she show me some book and she was right (I was right too, in Argentina we write the division differently!)

I had that too - the notation is different across countries. I had an even further problem, in that the Serbian mathematicians in XIX century translated (mostly literally, and very successfully) all of the Latin in the maths.