I have a 4" cast iron-to-PVC pipe vertical pipe connection that won't stop leaking. I'm using a banded Fernco No-Hub connector that says it is for CI-to-plastic connections but the OD on the CI pipe is 4" and the PVC is 4.25" and the gasket is way oversized. I even tried sleeving the smaller CI pipe with another gasket to make up the size difference to no avail. The pipes are fully supported so they are not moving and the coupling is snugged tight. Any suggestions on another method or a particular brand that is designed for a 1/4" transition?

Those rubber fernco clamps are actually not designed for above ground plumbing work... moreso designed for underground plumbing so no huge surprise you have a problem.

In this case, when service weight cast iron is in place you may be better off purchasing a 4-band clamp like in the picture below. These will only be avialable at a local plumbing supply house.

You could also purchase a 4" no-hub x 4" PVC adapter. This adapter transitions from cast iron to PVC and you use the same 4 band clamp I showed you below. Here, adapter gets cemented onto PVC and then cast iron gets clamped to the other side of the adapter... does great job making up difference between pipe sizes... ;)

Let me know if you have more questions...

MARK