You would sell at a discount.
Assume you invested $1000 in the treasury bond
Create the cash flow stream of 59 periods of $20 (4% on $1000 semi-annually) and the final payment of $1020 which includes the interest and return of principal.
take the present value of this stream at 4% and you will get $1000
Now take the present value of the stream at 5% and you will get a value less than $1000. That would be the value and you can use it to figure out the discount.
Hope this helps.
Tom