Megan buys a bond that is redeemable for its par value of 20,000 after 5 years. The bond pays coupons of 800 annually. The bond is bought to yield 8%...

Megan buys a bond that is redeemable for its par value of 20,000 after 5 years. The bond pays coupons of 800 annually. The bond is bought to yield 8% annually. Calculate the accumulation of discount (principal) in the 4th coupon. 

Please provide step by step process and not solutions from excel. Thanks.