The Flex-A-Spout, like most extenders, is compatible with both 3-inch-long by 2-inch-wide and 4-inch-long by 3-inch-wide ...

You’ll need a downspout adapter and end cap, some 4-in. PVC pipe and adapters. Pros: This is especially effective when the extension needs to cross a walkway or is in a “knock-off-prone” area.