Fernco 1056-64 1056-64 6 in. x 4 in. Flexible PVC Coupling

That reducing step is exactly what the 1056-64 is sized for: 6 in. on one end, 4 in. on the other. It's a flexible PVC coupling, so the transition is made with the coupling sleeve and clamps rather than a solvent-welded reducer bushing, which is what makes it workable in a repair where the two lines don't line up perfectly. Match the pipe outside diameters to the coupling ends before you order, since coupling sizing follows the pipe OD, not the nominal call-out alone.

Order it as Fernco 1056-64. That's the manufacturer part number for the 6 in. x 4 in. flexible PVC coupling, and it's the number to quote to a distributor so you get the reducing size rather than a straight 6 x 6 or 4 x 4.

A flexible coupling like the Fernco 1056-64 is the choice when you're cutting into existing pipe in a trench or a wall and can't spin or dry-fit a rigid reducer into place. It clamps over both pipe ends, so it tolerates a small amount of misalignment and lets you get the joint apart later. A solvent-weld reducer is the choice for new PVC work you're building up on the bench, where the joint will be permanent and the run is straight. Both do the 6 to 4 transition; the difference is whether you can build the joint rigid or have to close a gap in pipe that's already in the ground.