I sent a check for 4th quarterly estimated tax payment to IRS and incorrectly dated it for Jan. 6, 2019 instead of 2020. Will IRS cash it if dated over a year late?

No one can predict if they will notice it.  Writing the wrong year is a common error at the very beginning of the year, so they might not even notice it.  You are just going to have to watch your bank account and see what happens.