What happens if someone tries to pay $2,000 twice instead of $4,000 once?  Does the clerk have real-time access to all payments made on the account, and for which quarter they were applied?  There once was a time when IRS revenue officers would require monthly ES payments from delinquents with a long history of recidivism.  

How much is the penalty rate in Virginia?  If it's the same as IRS, 3% until recently, your client is a poor investor if she hasn't been making more than that with her own funds.  Right there in Virginia, Altria pays a dividend of more than 6%.  (Yes, I know, that's a pre-tax yield.)