If I missed paying estimated taxes for the first 3 quarters of 2020, should I send ONLY 4th quarter amount in now, or send in paymts for all 4 quarters?

[Critter-3]

Your only choice right now is to send in one 4th quarter payment ASAP.   If you send in 4 separate payments at the same time it is exactly the same as one payment now. 

[dmertz]

Assuming that you have underpayments for quarters 1 through 3 with respect to the safe-harbor provisions, the earlier you make up for the underpayments, the less penalty and interest you'll owe.  To add to what Critter-3 said, you'll likely want to make one estimated payment now covering the entire total that should have been paid for the 4 quarters of the year, not just the amount that would have been required for Q4 had you timely made payments for Q1 through Q3, otherwise your penalty and interest will be higher than necessary.

[jcfleming]

Thank you both!

[MSCO9241]

if you want you can pay directly online using IRS direct pay

https://www.irs.gov/payments