Fernco 1056-54 1056-54 5-in.x4-in. Flexible PVC Clamp Clamp Reducer Coupling, Black

That's exactly what the 1056-54 does. It's a reducing coupling, 5 inch on one end and 4 inch on the other, so it takes a 5 inch line down to 4 inch in a single fitting instead of needing a separate reducer plus two straight couplings. Both ends are clamped, so the pipe slips into the flexible sleeve and the clamps tighten down over it.

Reducing couplings of this style are used for transition joints because the seal is made by the clamped sleeve gripping the outside of the pipe, not by matching a hub or a thread. The practical check is outside diameter, not material: the 5 inch end has to fit the OD of your larger pipe and the 4 inch end the OD of your smaller one. Different materials in the same nominal size don't always share an OD, so measure both pipe ends before you order.

A flexible clamp coupling is what you reach for when the two pipes aren't perfectly aligned, when you're tying into existing buried or in-wall pipe you can't rotate, or when you want the joint to come apart later. The 1056-54 is a rubber-style sleeve clamped over both pipe ends rather than a glued or hubbed connection. Solid PVC hub fittings are the better choice inside a new, fully solvent-welded DWV run where everything lines up and the joint is meant to be permanent.

No solvent cement, no primer. The 1056-54 is a mechanical joint: the pipe ends slide into the flexible sleeve and the clamps at each end are torqued down to seal it. That's the practical reason plumbers and remodelers use one on a repair, you can make the connection on wet or dirty pipe and in a trench where you can't get a glued joint to set.

Order it as Fernco 1056-54. That number identifies the 5 inch by 4 inch reducing configuration specifically, which matters because the 1056 family runs across a range of size combinations and the suffix is what pins down which reduction you're getting.

The 1056-54 is the unshielded style: a black flexible sleeve with a clamp at each end, no full-length stainless shear ring over the body. Unshielded couplings are the common choice for gravity DWV and no-hub repair work. If your inspector or the local code calls for a shielded coupling to resist shear on a buried or unsupported joint, that is a different part and you want a banded model instead.