"I recently learned that the Q4 estimated tax payments are not due until Jan 15, 2021, and am trying to figure out if I will owe any penalty for underpayment / no payment of estimated taxes."
That's the due date for the last of 4 estimated tax payments for 2020. The estimated tax due dates for 2020 were April 15 – extended to July 15, 2020, June 15 - Extended to July 15, 2020, and September 15, 2020. Since you earned your $100K pretty much entirely in the 1st quarter of 2020, you should have been making payments at each of those dates, assuming that you needed to.
"Would the IRS assume that the income was evenly earned throughout the year?"
Yes, and that works to your benefit here. Instead of having income of $100K in the first quarter - meaning that every quarter the penalty will be based on $100K earned "year to date", the penalty will be calculated on $25K in the first quarter, $50K in the second quarter, and so on.
"Would I owe estimated taxes for 2020?"
Almost certainly. There are instances where you wouldn't need to pay estimated taxes, e.g., this was the first tax return you were ever required to file, but that's not the case here. Figure out a rough estimate of taxes owed - income taxes and SE taxes - and pay it as soon as you can. I can't think of any way of avoiding the penalty.
