CB40M

Medium density puts it right in the middle ground between a firm ball that might not conform well to pipe walls and a soft one that compresses too easily under pressure. For general cleanout use, that balance tends to work well because the ball maintains enough shape to push debris through while still flexing where it needs to. The elasticity holds up through repeated use, so it won't go flat or misshapen after a few passes.

This medium density sponge ball is designed to withstand substantial pressure while still retaining its shape, so it should perform reliably at that pressure level.

For 4 inch cleanout work, the CB40M is the medium density sponge ball. That medium density build is meant to give a good mix of elasticity and durability for routine cleaning.