What is (x+y)^4???????????/

(x+y)^4 = (x+y)² × (x+y)² = (x²+2xy+y²)(x²+2xy+y²) = x⁴+4x³y+6x²y²+4xy³+y⁴.
If you know binomial theorem, it's faster.

((x + y)^2)*(x + y)^2 = (x^2 + 2*x*y + y^2)*(x^2 + 2*x*y + y^2) = x^4 + 4*x^3*y + 6*x^2*y^2 + 4*x*y^3 + y^4

x^4 + 4x^3y + 6x^2y^2 + 4xy^3 + y^4