The CP504 is simply the second to last letter you get before the IRS starts collection activity. Your case will later probably be assigned to the IRS Automated Collections System or it may be with a Revenue Officer if you got a high balance.
Watch out for the CP90 letter coming up next
30 days after the letter’s date, the IRS will typically send out another letter, the CP90, also known as the final notice of intent to levy. 30 days after the CP 90 garnishments for wages, levies on 1099 sources, and levies on bank accounts are going to start getting sent out.
So right when you get the CP504 letter it is a good time to propose a resolution if you did not already to the IRS.
If you are doing your own case check out our free tax help guide.