You can save four designs for later use.
When in Designer Studio click on "Save Design" button. This will save your design in "Saved Designs" section which can be found in "My Account" page.
To retrieve "Saved Designs" go to the "My Account" page and click the add(+) button. It will automatically load into Designer Studio for you to add to a shirt.
If you click "Add to Store" button Designer Studio will not allow designs to load after the max has been reached.