The four screws (2 per side) are the screws you need to remove to take the lights out without removing the bumper. Plus the nuts, (four a side on the backs of the lights) that have to be removed, before you can pull the taillights out. Take the two screws out from underneath on both sides.
To get the two screws out of each taillight you'll need a long extension for your ratchet, about 14" long. Becareful that you don't push down on the top of the bumper support bracket when the tails are out. If you bend it down the bumper will not fit tight against the tails when you re-install them or the new lights.