4 inch gutters in my area are constructed with a 3 inch round down spout, which feeds into a 3 x4 square gutter down pipe. Wind and weather can cause the pipe to disconnect since they are loosely fitted.  This adapter fits over the down spout. The down pipe is then fitted over the adapter and secured with a self taping screw through the pipe and into one of the adapter tabs. The adapter can be optionally glue to the down spout.