Q. Any issue/auditing concern?
A. No. The IRS doesn't care if you overpay. Yours is a common situation, so it does not raise a red flag.
Q. Any good strategy going forward to avoid a huge swing from year to year?
A. Not really, it's a guessing game.
In the example you gave; once you knew you were having an income drop, it was not necessary to make the 4th quarter payment (or you could reduce the amount) once you knew your revised total (estimated) tax for the year.
