I believe that they are claiming 4.5GW of _capacity_, not 4.5GWh of production. These are vastly different numbers.
As a rough estimate, figure that on average you only use about 1/3 of your capacity, since the wind doesn't blow all the time. There are 8766 hours in a year, so 4.5GW of capacity give you about 1.3*10^10 kWh per year. That is real money.
As I understand it, wind power is currently approximately competitive with new gas turbine power production...but only if the interest rates are right, and the life of the turbines is long enough to amortize the investment, and fuel costs stay high. With wind you put most of the money in up front as capitol investment; with gas turbines you pay for the fuel. Given how much fuel costs, you can see how much this must mean wind power costs _up front_ if the two are roughly competitive.