Preheat the oven to 350. On a baking sheet, toss the bread cubes with 1 tablespoon of the olive oil and toast for about 10 minutes, stirring once, until golden. Let the croutons cool. In a large bowl, whisk the mustard with the balsamic vinegar. Gradually whisk in the remaining 3 tablespoons of olive oil and season with salt and pepper. Add the romaine, cucumber and croutons, toss well and serve.