We use the rule that if y = x^n then dy/dx = n*x^(n-1) which is valid whether or not n is an integer.
We also use that differentiation is a linear operation, which means that we can differentiate term by term in the expression for y.
Noting that 3 = 3*x^0, we therefore have
dy/dx = 4*x^3 + (1/3)*x^(-2/3) + 0