You don't have to attach a copy, but you do have to enter the information from your T4 (and other) slips.

You can use Auto-Fill to add your slips. Auto-Fill will be available tomorrow.

Or you can enter your slips manually. To do this in TurboTax Online, select Income>T-Slips>T-Slip Summary from the left side menu. Select the slips you need, then click the “Continue” button to go through the pages.