sure, that can be done.
add a new column for your option, for the sake of example, lets call it "itemText" set the unique attribute for this new column. in the add to cart button, set the itemText column to updatable from a text box.
now in the eCart object, go to the calculations tab, create a new calculation called realPrice:
(abs([itemText]!='')?[Price] + 4:[Price])
and update the total Price calculation to:
(abs([itemText]!='')?[Price] + 4:[Price]) * [Quantity]
when you create the cart display page, on the second page of the cart display wizard, remove the column for price, and add the realPrice column in it's place.