Have you been filing tax returns during those years? The w4 is used to determine the amount that is withheld from your paycheck, and then your tax is reconciled on your tax return. There would be no additional refund.
No. And it makes no difference. Your W-4 only tells the employer how much tax to withhold from your paychecks. It only goes to the employer. In January your employer gives you a W-2 to show how much income you received and how much tax was withheld. Your refund or tax due is calculated based on those amounts. The software uses the filing status that you entered when you prepared your tax return---so if you entered that you were filing as married on your tax return, you were taxed correctly.
If you want to change your W-4 going forward:
https://turbotax.intuit.com/tax-tools/calculators/w4/
https://www.irs.gov/individuals/tax-withholding-estimator
https://www.irs.gov/pub/irs-pdf/fw4.pdf
If they took out more withholding for Single than for Married you would get any refund on your tax return. I’m married but claim Single on my W4 to have more taken out. The withholding is just an estimated amount to cover the tax on your total income including your spouse’s income and interest etc. It probably worked out better having more withholding taken out.