APV discount rate – Amberle CO (Dec 18)

Dear Sir,
Hope you are doing well. I have a question about this exercise.

In the answer there is written that the tax shield can be discounted at 8% or 3.1 % (government bond )

would it be correct to discount at the risk free – rate of 4%? or in this case we can use only 8% and/or 3.1%

thanks for your feedback,
regards,
Marco