4. (8 points) A poplar tree, a willow tree, a locust tree, a birch tree, and a sycamore tree are planted in a row, with the distance between adjacent trees being 1 meter. The distance between the poplar tree and the willow tree, and the locust tree is the same, and the distance between the birch tree and the poplar tree, and the locust tree is the same. Therefore, the distance between the sycamore tree and the birch tree is $\qquad$ meters.
【Answer】Solution: The distance between the poplar tree and the willow tree, as well as the locust tree, is equal, so the positions of all three trees could be: Willow Poplar Locust, Willow Poplar Locust $\square$, $\square$ Willow Poplar Locust $\square$, Willow Poplar Locust, where $\square$ represents an unknown position. Since the birch tree is equidistant from the poplar tree and the locust tree, the only possible arrangement is: Willow Poplar Birch Locust, with the remaining position being the sycamore tree.

Therefore, the distance between the sycamore tree and the birch tree is 2 meters.
So the answer is: 2.