That makes sense. I might be making the situation more complex than it is, but I was viewing this as more of the following:
Currently there is 500K debt at 4%. By paying 140, the rate on the remaining 360 drops by up to 1%. So, the equivalent on the upside would have to be 4% * 140 + 360*1%.
If I am thinking about this right, it works out to about 6% total return on the repayment - with locked funds and guaranteed return (there will be some additional loss from transaction costs).
I just wanted to see if I was missing something.