So you've just won hook a duck, for 9999 gold all because the goblin is going to offer a legendary modifier to your terra blade. So you've spent the gold instead of reforging for a 50:50 chance.
And now the goblin tinkerer gives you four boxes... 2 contain legendary modifiers, 1 contains broken and 1 contains useless.
You can only pick once, before having to repay 9999 gold and win hook a duck...
So, it's 50:50, and there's 2 legendary's, 1 broken and 1 useless modifier (you currently have no modifier on the terra blade so broken would still be horrible)
Choose wisely...