How do I get my federal income tax returns from the last 4 years?

Did you use TurboTax to prepare them?  You have to sign in to the exact account using the user ID that was used when you prepared the returns.

For 2018:

PRINT RETURN   You need to print a copy of your return?  Sign in to your account using exactly the same account and user ID that you used to prepare your return.
On the bottom of the screen find Your tax returns & documents and click on Show.  Click on the Year and Click on Download/print return (PDF)

Access to prior year returns

https://ttlc.intuit.com/questions/3355524-when-does-access-to-my-prior-year-turbotax-online-returns-expire

https://ttlc.intuit.com/questions/3404575-how-do-i-access-my-prior-year-2017-return

You can get a free transcript from the IRS:   

https://www.irs.gov/individuals/get-transcript

How do I get a copy of a return I filed using online TurboTax?

https://ttlc.intuit.com/questions/1900748

How do I get a copy of a return I prepared using desktop TurboTax?

https://ttlc.intuit.com/questions/1900639

Note:  Every year, before mid-October, print a copy of your current year tax return and save it as a tax file and pdf, too.  That way you will have it when you need it for school, a loan, next year’s tax return, etc. without going through a lot of aggravation (and expense)  to get it.

https://ttlc.intuit.com/questions/1899645-how-do-i-save-my-2018-turbotax-online-return-as-a-tax-data-file

https://ttlc.intuit.com/questions/1912544-how-do-i-save-my-2018-turbotax-online-return-as-a-pdf

if you can't get them through TT  you can go online to irs website and get transcripts  either online or by filing form 4506-t. if you need actual copies you can get them by filing IRS form 4506 (not 4506-t - which is for transcripts only)