Final answer:
Sarah's dad should invest $43,200 into a savings account today to cover Sarah's rent for the next four years.
Step-by-step explanation:
To calculate how much Sarah's dad should invest into a savings account today, we need to determine the total amount she will need to pay for rent over four years. Since rent is $900 payable at the beginning of each month, the total rent amount for four years would be: 900 * 12 * 4 = $43,200. Therefore, Sarah's dad should invest $43,200 into the savings account today in order to cover her rent for the next four years.