Lobster, Corn, and Potato Salad with Tarragon

Prepare lobster, potatoes, and corn:

Plunge 2 live lobsters headfirst into an 8-quart pot of boiling salted water. Cover and cook lobsters over moderately high heat 9 minutes from time they enter water, then transfer with tongs to sink to cool. Return water to a boil and cook remaining 2 lobsters in same manner. Leave water boiling in pot.

Simmer potatoes in lobster cooking water until just tender, 15 to 20 minutes. Transfer with a slotted spoon to a colander, reserving boiling water in pot.

Boil corn in same water until crisp-tender, about 3 minutes, then drain.

When lobsters are cool, remove meat from claws, joints, and tails, reserving shells for another use.

Cut meat into 1-inch pieces. Cut potatoes into 3/4-inch pieces, and cut corn from cobs.

Make vinaigrette:

Whisk together lemon juice, tarragon, mustard, and salt until combined, then add oil in a thin stream, whisking.

Assemble salad:

Just before serving, toss together lobster meat, potatoes, corn, vinaigrette, tomatoes, frisée, and scallion in a large bowl and season with salt and pepper.