In a medium saucepan cook green beans in boiling salted water 2 minutes. Drain and rinse under cold water; drain again.
Wipe out saucepan. Heat 1 Tbsp. oil over medium heat. Add couscous. Cook and stir 1 to 2 minutes or until lightly browned. Add 2/3 cup water and 1/4 tsp. of the salt. Bring to boiling; reduce heat. Simmer, covered, 10 minutes or until tender and water is absorbed. Remove; let stand 5 minutes.
Arrange couscous, beans, tomatoes, and jicama on a platter. In a small bowl whisk together lemon juice, capers, mint, parsley, remaining 1/4 tsp. salt, and 1/4 tsp. black pepper. Slowly whisk in remaining 3 Tbsp. oil. Pour over tomato mixture.