There is no easy way to drain the torque converter, or the cooler lines, etc. Having 7 quarts on hand will guarantee that you have enough, plus some extra.
Be sure you put the sump filter seal into the trans first, then install the filter. If the old one happens to come out with the filter, that's ok. Then, install the new one into the trans. You don't want to put it on the filter, and try to install it that way.
Be quite careful when you are tightening up the many pan bolts! They are easy to strip! The torque rating for them is in inch pounds. Add fluid slowly while checking for leaks. Once you get 4 fresh quarts in there, fire it up and top it off. Remember, trans fluid expands as it warms up, so don't overfill.