Blanch cauliflower florets until half-cooked, drain and set aside to cool.
Whisk together vinegar, lemon juice, garlic, and parsley. While whisking constantly, add oil in a thin stream. Season with salt and pepper. Chop 2 of the hard boiled eggs. Cut the other 2 egg into wedges for garnish. Toss cauliflower, chopped eggs and black olives together. Let marinate and chill for 2 hours. Transfer to a serving dish and garnish with egg wedges and parsley sprigs.