The temperature of the water should drop from the cold eggs. When it comes back up to a simmer, reduce the heat to maintain a very gentle simmer. If the water is boiling, reduce heat. Cook eggs 4 minutes for runny yolks.
Remove from the water with a slotted spoon to a plate lined with a clean towel.
Meanwhile, whisk cider vinegar, shallot, and Dijon in a large salad bowl. Whisk in oil, tarragon, salt, and pepper.
Add kohlrabi and endive and toss to coat. Divide salad among four dinner plates. Top with crumbled bacon and the hot poached egg.