I don't see an option called "Tax Tools" anywhere. When I log in, I can only see two options on the left: "Tax Home" and "Documents". There is nothing under "Documents".
The Form 4868 will be included in the PDF of your tax return after you have paid for the TurboTax online account.
Click on Tax Tools on the left side of the online program screen. Then click on Print Center. Then click on Print, save or preview this year's return. Choose the option Include government and TurboTax worksheets