My understanding is if you are expecting to owe more than 2k in taxes you are supposed to pay quarterly estimate taxes.
If you don't you might pay a penalty interest rate on the taxes owed to the IRS (Not sure how much that interest rate is...)
If you make a significant capital gain after the last quarter estimate payment due date do you still get that penalty? Since it is gained after the cutoff date for submitting estimated taxes I would think not but want to be sure.
I'm also not really sure how or where to go to e-file estimated taxes to begin with... I was trying to find it on the IRS website but could only find an explanation of when you have to pay estimated taxes.