Prepare rice according to package directions. While the rice is still warm, mix in the rice vinegar, sugar, and salt. Let cool slightly.
Place 1 nori sheet on a cutting board; spread 1/4 cup rice mixture evenly over nori, pressing slightly.
Arrange a few strips of cucumber, carrots, red and yellow pepper strips lengthwise down center over rice. Carefully roll into a log. Repeat with remaining nori sheets.
In a small bowl, whisk wasabi powder, cold water, soy sauce, and daikon together.
Cut each roll crosswise into 1-inch pieces. Place on platter cut-side up. Serve at room temperature. Serve with dipping sauce.