TurboTax Online works only for the current year (2017) taxes. So if you are needing to file taxes for 4 years prior, you will need to purchase the Desktop version for each year that you need to file.
Here's a link to access those products:http://shop.turbotax.intuit.com/personal-taxes/past-years-products.jsp
Also, here's a TurboTax FAQ on how you can file for previous years.
https://ttlc.intuit.com/replies/3301770
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"