Carl puts $10000$ into a bank account that pays an annual effective interest rate of $0.04$ for $10$ years. If a withdrawal is made during the first five and a half years, a penalty of $5\%$ of the withdrawal amount is made.Carl withdraws $K$ at the end of years $4$, $5$, $6$, $7$. The balance in the account at the end of year $10$ is $10000$. Calculate $K$.
From the effective interest rate, we get the compound interest rate to be $0.04$.
Then at $t=4$: $A(4)-K-0.15K=10000(1.04)^4-1.05K=11698.58-1.05K$
The, afterwards, I do not exactly know what to do.