I filed my return through turbotax (I owed over 4k this year). I received a notice from the IRS that we owe an additional $900. Any advice on how to find out why/appeal?

[xmasbaby0]

I RECEIVED A LETTER FROM THE IRS/STATE

TurboTax does not receive any information from the IRS or your state after your return was filed, so no one at TurboTax knows about a letter you received from the IRS or the state.

(what the letters from the IRS mean)

I GOT A LETTER FROM THE IRS   

https://ttlc.intuit.com/questions/3605673-i-got-a-letter-or-notice-from-the-irs

I AM BEING AUDITED

If you purchased Audit Defense:

https://ttlc.intuit.com/questions/2924451-i-bought-max-or-audit-defense-and-now-i-m-being-audited

AUDIT SUPPORT

TurboTax has a special phone number for help with IRS/state tax letters, which will be easier than going through regular Customer Support.   At the page below, click on the blue button that says "Get Help from TurboTax Support".  Then it will ask you what tax year is your letter.  Then it will ask you what the letter concerns. Then it will show you the phone number during posted business hours.

https://support.turbotax.intuit.com/irs-notice/audit-support/

[rjs]

If this is the first notice you have received, the notice should tell you why you owe the additional money, and what to do if you disagree. Read the entire notice carefully.

What is the notice number or letter number at the top right, starting with CP or LTR? Is it about your 2019 tax return?

You haven't given any details, but I'll take a guess at what might be going on. If you owed such a large amount when you filed your tax return, the IRS might be billing you for penalties and interest for late payment of the amount that you owed. You should have been making payments throughout the year, by withholding or estimated tax payments, instead of waiting to pay it all when you filed your tax return.