Center one sheet nori on a bamboo sushi mat. Wet your hands. Using your hands, spread a thin layer of rice on the sheet of nori, and press into a thin layer. Arrange 1/4 of the cucumber, ginger, avocado, and imitation crabmeat in a line down the center of the rice. Lift the end of the mat, and gently roll it over the ingredients, pressing gently. Roll it forward to make a complete roll. Repeat with remaining ingredients.