Stephanie purchased a machine for $$21,000$ for his company. He paid $5\mathrm{.}00\%$ of this amount as a down payment and financed the rest at $5\mathrm{.}92\%$ compounded monthly. He paid $$1,125$ at the end of every month to settle the loan.

a$.$ What was the principal portion of payment number $4?$

Round to the nearest cent

b$.$ What was the interest portion of payment number $4?$

Round to the nearest cent