We test gravity sanitary at 3 lbs of pressure and ferncos will hold that pressure, I don't think they would take much more. You mentioned going from 6" to 4" , I hope the pipe is getting bigger towards the street/sewer main.If it's being reduced and it's leaking there try adapting from clay to pvc (with the fernco)and then using an eccentric 4 to 6 adaptor and then convert back to clay with another fernco. If you did everything right there should be no leaks.BTW we use gasket joint pvc pipe at work,if you use glue-joint make sure you have the right glue and primer.
But like Medmech said it should be no big deal, if it's leaking at the fernco then the sealing surface (probably on the clay) isn't clean or the clamp isn't tightening against the rubber/clay.If the clay pipe has a rough end it can be squared up with a crescent wrench sized to the pipe thickness and used to slowly 'nibble' away at the clay. Good luck

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