Toast 4 slices of bread until golden and crisp, then arrange them on a large clean serving platter.
In a small bowl, whisk together 1/2 cup Greek yogurt, 1/2 teaspoon salt, and 1/2 teaspoon prepared wasabi paste until smooth; taste and adjust seasoning if desired.
Spread about 1 tablespoon of the wasabi-yogurt mixture evenly over each slice of toasted bread.
Top each tartine with an even portion of smoked salmon (total 8 ounces), then scatter sliced radishes and the chopped herbs (cilantro, scallions, parsley, and dill) over the salmon.
Squeeze fresh lemon juice over the tartines and sprinkle lemon zest, then season lightly with salt and freshly ground black pepper to taste.
Cut each tartine in half if desired and serve immediately.