Just use the last four, if it's an option. I print the 8879, make a copy and white-out the SS, and all PINS, including my ERO PIN and send that to my out of state clients. I attach the signed one to the 8879 with the numbers and put them in the file. But I'm still old school and most of my clients sign in person.

Unless the client contacted the IRS and filed a complaint about you e-filing them contrary to their instructions, you're safe on any 8879 requests from the IRS.

If you are facing an audit, the SS on the 8879 is probably going to be the least of your concerns, in my experience.