Use a sharp knife or kitchen scissors to cut the crab into 3” to 4” long pieces, splitting thicker pieces in half, if desired. Preheat the barbecue to high. Tear off four 18” x 12” sheets of tinfoil. Brush the centre of each sheet with an equal amount of olive oil. Place an equal amount of crab in the centre of each piece of foil. Drizzle the crab with the lemon juice. Top with small nuggets of butter. Pull up the sides of the foil to loosely cover the crab; crimp at the top to seal. Set the foil packages on one side of the barbecue, close the lid, and turn the heat off under the crab. Leave the heat on the other side of the barbecue set to high. Cook for 8 to 10 minutes, or until just heated through. Garnish portions of the crab with a lemon slice or two.