I'm over 70 - filling as a single - have only SS income and less than $3,000 in other income. Do I need to file? Also, SS took $4,300 in voluntary Fed Tax withholding

You should file the tax return to get a refund of the federal taxes withheld.  You should also contact Social Security and stop the voluntary federal taxes from being withheld from your benefits.

To enter Social Security benefits reported on form SSA-1099

• Click on Federal Taxes (Personal using Home and Business)
• Click on Wages and Income (Personal Income using Home and Business)
• Click on I'll choose what I work on (if shown)
• Scroll down to Retirement Plans and Social Security
• On Social Security (SSA-1099, RRB-1099), click the start or update button

Or enter social security benefits in the Search box located in the upper right of the program screen.  Click on Jump to social security benefits