whether or not you need to pay Q4 ES depends on your 2024 tax liability net of this credit which isn't clear from your post. If you expect a refund in April, then you probably don't want to give Uncle Sam more interest free money for 3 months. If you still owe tax you need to have met the estimated tax requirements (90% of 2024 tax owed, or 100% of 2023 (110% if AGI > 150k) whichever is smaller. Not sure how a big solar tax credit affects timing if you had uneven tax liabilities or uneven estimated tax payments thru the year (Form 2210 is used to sort that out). If unsure about any of this or the tax credit amount ("should be a big tax credit") suggest running a draft 2024 return in TT with high level totals to see credits/tax/penalty outcome, this is what I do to line up final ES payment and minimize penalty if I messed up.
See more info on requirements see https://www.irs.gov/faqs/estimated-tax
This thread was also helpful in terms of how to think about the 'safe harbor' requirements thru the year
https://ttlc.intuit.com/community/taxes/discussion/re-irs-safe-harbor-rule-for-tax-payments/01/34171...
Not an CPA/expert just my 2 cents - hope this helps and finds you safe in CA.
