Hi, yvonne1777, happy to address any follow up questions you have!
First, you are correct - the self employment taxes only relate to the federal income tax return; California does not have a self-employment tax on their income tax return.
Second, the IRS generally presumes that income is earned ratably throughout the year, so they want the estimated tax payments ratably throughout the year as well. So, if a person pays most/all of the estimated tax payments in the latter part of the year, the IRS can assess a penalty for not paying in enough during each quarter of the year. Even if you were to get a refund, you can still be assessed a penalty. For example, let's say someone is self-employed and only made the fourth quarter estimated payment, but made sure to make the amount large enough to generate a refund. Even though when they file their return there is a refund, the IRS can assess a penalty for not paying in during Quarters 1, 2 and 3.
NOTE: there is a way to pay in more taxes later in the year AND not be penalized for doing so. If your income is not earned ratably throughout the year, you can pay quarterly payments based on the actual amounts earned each period. When you complete your tax return, you will have to complete a schedule showing how the income was earned in different quarters and how the payments made in each quarter were enough to cover the tax liability for that quarter. So, in your situation, if your business earned 25% of its income in the third quarter and 75% of its income in the fourth quarter, you can make the estimated tax payments lower for the third quarter and higher in the fourth quarter, then complete form 2210 of your tax return to document that the withholdings corresponded to the quarters when the income was earned.
If, however, this is not the case for you, then I would suspect the IRS will charge you for not making a big enough payment in the third quarter.
As a final comment, even if you are not able to pay the full amount of your third quarter estimated payment amount, you should pay as much as you can by the due date of the estimated payment, to reduce potential penalty amounts.
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