If it is a $400 voucher, the answer is yes. If it is a "free flight coupon", as you list in the heading, the answer is no. But I'm thinking because you mention a dollar value, then that is what you have. In that case, the $400 voucher works just like money against the fare and you get miles just as if you'd paid for it. Free flight TCVs are far more difficult to book because they come out of the same bucket as standard award tix. Those "W" class tickets are severely capacity controlled and they don't earn miles.